Ion implantation system and control method

ABSTRACT

An ion implantation is disclosed that includes an ionization chamber having a restricted outlet aperture and configured so that the gas or vapor in the ionization chamber is at a pressure substantially higher than the pressure within an extraction region into which the ions are to be extracted external to the ionization chamber. The vapor is ionized by direct electron impact ionization by an electron source that is in a region adjacent the outlet aperture of the ionization chamber to produce ions from the molecules of the gas or vapor to a density of at least 10 10  cm −3  at the aperture while maintaining conditions that limit the transverse kinetic energy of the ions to less than about 0.7 eV. The beam is transported to a target surface and the ions of the transported ion beam are implanted into the target.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/365,719, filed on Mar. 1, 2006, which is a continuation of U.S.patent application Ser. No. 10/433,493, filed Jan. 8, 2004, now U.S.Pat. No. 7,064,491, which is a U.S. national chase of PCT ApplicationNo. PCT/US01/18822, filed Jun. 12, 2001, which is a continuation of PCTApplication No. PCT/US00/33786, filed Dec. 13, 2000, which claimspriority and benefit of U.S. Provisional Patent Application No.60/267,260 filed on Feb. 2, 2001, U.S. Provisional Patent ApplicationNo. 60/257,322 filed on Dec. 19, 2000, U.S. Provisional PatentApplication No. 60/250,080 filed on Nov. 30, 2000, and is acontinuation-in-part of U.S. patent application Ser. No. 09/736,097,filed Dec. 13, 2000, now U.S. Pat. No. 6,452,338.

TECHNICAL FIELD

This invention relates to ion sources, implantation, and moreparticularly to ion implantation with high brightness, low emittance ionsources, acceleration-deceleration transport systems and improved ionsource constructions.

BACKGROUND

The following patent applications, herein incorporated by reference,describe the background of this invention: Provisional PatentApplication Ser. No. 60/261,260, inventor Thomas N. Horsky, filed Feb.7, 2001, entitled Ion Source for Ion Implantation; Provisional PatentApplication Ser. No. 60/251,322, inventor Thomas N. Horsky, filed Dec.19, 2000, entitled Ion Implantation; PCT Application Serial NumberUS00/33786, inventor Thomas N. Horsky, filed Dec. 13, 2000, entitled IonImplantation Ion Source, System and Method and filed Nov. 30, 2000,having the same reference. The referenced patent, for U.S. Purposes, isa continuation in part of my U.S. Provisional Application 60/170,473filed Dec. 13, 1999 60/170,473, now expired.

Background: Ion Implantation

Ion implantation has been a key technology in semiconductor devicemanufacturing for more than twenty years, and is currently used tofabricate the p-n junctions in transistors, particularly for CMOSdevices such as memory and logic chips. By creating positively-chargedions containing the dopant elements (for example, ⁷⁵As, ¹¹B, ¹¹⁵In, ³¹P,or ¹²¹Sb) required for fabricating the transistors in, for example,silicon substrates, the ion implanters can selectively control both theenergy (hence implantation depth) and ion current (hence dose)introduced into the transistor structures. Ion implanters havetraditionally used ion sources which generate ribbon beams of up toabout 50 mm in length; these beams are transported to the substrate andthe required dose and dose uniformity is accomplished by electromagneticscanning of the ribbon across the substrate, mechanical scanning of thesubstrate across the beam, or both.

With the recent advent of 300 mm-diameter silicon substrates in chipmanufacturing, there has been a keen interest in producing ribbons oflarger extent than has heretofore been possible with conventional ionimplanter designs, in order to increase wafer throughput when usingthese larger substrates. Taller ribbon beams enable higher dose rates,since more ion current can be transported through the implanter beamline due to reduced space charge blowup of the extended ribbon beam.Many of these new implanter designs also incorporate a serial (one waferat a time) process chamber, which offers high tilt capability (e.g., upto 60 degrees from substrate normal). The ion beam is typicallyelectromagnetically scanned across the wafer, which is mechanicallyscanned in the orthogonal direction, to ensure dose uniformity. In orderto meet implant dose uniformity and repeatability specifications, theion beam must have excellent angular and spatial uniformity (angularuniformity of beam on wafer of <1 deg, for example). The production ofbeams possessing these characteristics imposes severe constraints on thebeam transport optics of the implanter, and the use of large-emittanceplasma-based ion sources often results in increased beam diameter andbeam angular divergence, causing beam loss during transport due tovignetting of the beam by apertures within the beam line. Currently, thegeneration of high current ion beams at low (<2 keV) energy isproblematic in serial implanters, such that wafer throughput isunacceptably low for certain low-energy implants (for example, in thecreation of source and drain structures in leading-edge CMOS processes).Similar transport problems exist for batch implanters (processing manywafers mounted on a spinning disk), particularly at low beam energies.

While it is possible to design beam transport optics which are nearlyaberration-free, the beam characteristics (spatial extent and angulardivergence) are nonetheless determined to a large extent by theemittance properties of the ion source (i.e., the beam properties at ionextraction which determine the extent to which the implanter optics canfocus and control the beam as emitted from the ion source).Arc-discharge plasma sources which are currently in use have pooremittance, and therefore limit the ability of ion implanters to producewell-focused, collimated, and controllable ion beams.

Background: Ion Implantation Sources

The standard ion source technology of the implanter industry is theEnhanced Bernas source. As illustrated in FIG. 1, this is an arcdischarge source which incorporates a reflex geometry: a hot filamentcathode immersed in the ionization chamber (where the dopant feed gasresides) emits thermionic electrons confined by a magnetic field, andare reflected from an anticathode located at the opposite end of thechamber. Thus, the electrons execute helical trajectories between thecathode and anticathode, and generate a high-density plasma (on theorder of 10¹² ions/cm²). This so-called “plasma column” is parallel toan ion extraction aperture slot from which the ions are extracted bybeam-forming optics. By generating a high-density plasma and sustainingdischarge currents as high as 10 A, the Enhanced Bernas sourceefficiently dissociates tightly-bound molecular species such as BF₃.However, the emittance of this source is large due to the followingplasma-related effects:

-   -   1) The plasma potential (typically about 5 V) introduces a        random component of velocity to the ions, which directly        translates into increased angular dispersion of the extracted        ions.    -   2) The temperature of the ions and electrons within the plasma        can reach 10,000 K, introducing a thermal velocity which adds to        (1), and also introduces an energy spread of several eV to the        ions (according to a Maxwell-Boltzmann distribution), making the        beam exhibit chromatic aberrations.    -   3) Coulomb scattering between the ions in the plasma introduces        an additional non-thermal spread in the ion energy.    -   4) A high extracted current density is needed due to a        predominance of unwanted ions (i.e., fragments such as BF⁺, BF₂        ⁺, and F⁻ in a BF₃ plasma), increasing space-charge forces at        extraction and causing emittance growth.    -   5) The presence of a strong magnetic field, required for        operation of all arc discharge sources, causes beam deflection        and hence further emittance growth of the extracted ion beam,        especially at low beam energy.    -   6) High-frequency noise present in the plasma is propagated into        the beam as high-frequency fluctuations in beam current and in        beam potential. This time-varying beam potential makes charge        compensation in the beam plasma difficult to maintain, since it        can cause a significant steady or even abrupt loss of the        low-energy electrons which normally orbit the beam (being        trapped by the positive beam potential), leading to space-charge        blowup of the ion beam.    -   7) The ion extraction aperture cannot be significantly elongated        beyond, say, 75 mm (typical length is between 20 mm and 50 mm),        since this requires a significant elongation of the plasma        column. Bernas sources become unstable if the separation between        cathode and anticathode is large, and larger cathode-anticathode        separations requires a much higher arc discharge current in        order to maintain a stable plasma, increasing power consumption.        Background: Ion Deceleration

Ion implanters of conventional design exhibit poor transmission oflow-energy boron at energies below a few keV, with the result that theseboron beam currents are too small to be cost-effective in manufacturingsemiconductor chips using sub-0.18 micron design rules. Next-generationimplanters which have been long-in-planning, and which were introducedinto the capital equipment market within the last few years incorporatea different principle of ion optics, attempting to solve this low-energytransmission problem. To counter the effects of space charge repulsionbetween ions, which dominates beam transport at low energies, aso-called “decel” (i.e. deceleration) approach has been developed toallow the ion beam to be extracted and transported through the implanterat a higher energy than the desired implantation energy so that spacecharge effects are not so detrimental, and by introducing a decelerationstage late in the beam-line, but upstream from the wafer target,reducing the ions to the desired implant energy as the ions approach thewafer target. For example, the ion beam can be extracted and transportedat 2 keV, but decelerated to 500 eV before the ions reach the wafer,achieving a much higher beam current than is obtainable withspace-charge-limited beams in beam lines of a conventional,non-deceleration design. Unfortunately, this method of employingdeceleration still has posed significant problems which have detractedfrom its production-worthiness. As the ion beam passes through thedeceleration lens to the wafer, the ion beam becomes spatiallynon-uniform to a great degree, and the ions impact the substrate with awide distribution of angles of incidence relative to the wafer surface,with potential so-called channeling effects. The spatial and angulardose uniformity of a decelerated beam is typically much worse than inconventional, non-deceleration ion implantation. This makes it difficultto achieve a uniform dose, and introduces the need to take other stepswhich affect cost and throughput. Compounding the problem is the factthat the grossly non-uniform profile of the ion beam also interferessignificantly with accurate dosimetry of the implant, since ionimplanters typically sample only a portion of an ion beam at or behindthe plane of the wafer. Dosimetry is used to control the degree ofimplant within a desired range. The accuracy problems with dosimetryproduced by partial sampling of a severely extended and non-uniformdistribution of ion current in the beam of an acceleration/decelerationimplantation system thus also affects the accuracy of the implant, thecapital cost of the implant system, the quality of the wafers, andthroughput of the system.

Another, quite different approach for shallow, low energy implants hasbeen proposed (but not implemented in current production) it is that ofusing molecular ion beams (having clusters of the dopant atom ofinterest) in conventional implanters that do not have a decelerationstage. Decaborane is one example of such a molecular material.

Chip manufacturers are currently moving to 300 mm-diameter siliconsubstrates for fabricating Complimentary Metal-Oxide-Semiconductor(CMOS) memory and logic chips to reduce manufacturing costs over thatattainable with 200 mm substrates. Though such a shift in wafer sizerequires building new factories populated with new semiconductormanufacturing tools for processing the larger-diameter wafers, thepotential cost reduction per die is about a factor of two. Thus, theexpenditure of billions of US dollars for these facilities has beenhoped to enable lower-cost manufacturing, and ultimately a hugecompetitive advantage for volume manufacturing of both commodity andleading-edge semiconductor chips. Such a cost reduction can only befully realized if the throughput of wafer units of the fab tools (thetools of the fabrication facility) is the same for 300 mm as 200 mmsubstrates, which had been to some extent been assumed would be thecase. Unfortunately, in the case of ion implantation to fabricateultra-shallow (and ultra high density) semiconductor junctions, even thelatest acceleration/deceleration implanters continue to bedose-rate-limited in their wafer throughput, so that there has beenessentially little or no net increase in productivity of semiconductordies by use of the larger wafers. This is a potentially difficultsituation for the chip manufacturer: if many more implanters must be putinto production to make up for their reduced output, the potential costreduction per die sought by use of the larger wafer geometry cannot berealized due to the increased cost of performing these critical implants(more investment in capital equipment, fab floor space, maintenancecost, etc.).

Background: Ion Doping

Over the last decade, implantation systems have been developed for theion implantation of very large substrates from which flat-panel displaysare manufactured. These “Ion Doping” systems deliver long ribbon ionbeams to the glass or quartz substrates, which are typicallymechanically scanned across a stationary ion beam. The substratedimension can be as large as a meter, and so the ion ribbon beam mustlikewise be long enough to ensure uniform doping (typically wider thanthe substrate). In order to generate such long ribbon beams,large-volume “bucket” sources are used. Bucket sources in a rectangularor cylindrical geometry are chambers surrounded by an array of permanentmagnets which provide magnetic confinement for the enclosed plasmathrough the creation of cusp magnetic fields. The plasma is generated byone or more RF antennas which couple RF power to the plasma. Anextraction lens forms the ribbon beam from the large-diameter source.

Because of the size of the ion doping system, mass analysis is not used,therefore all ion species created in the bucket source are transportedto and implanted into the substrate. This creates many process-relatedproblems including variations in ion implantation depth, and also theimplantation of unwanted species. Bucket sources are also particularlysusceptible to the accumulation of deposits within their largeionization volume, hence the potential of severe cross-contaminationbetween n- and p-type dopants requires the use of dedicated-use iondoping systems: the user must purchase one tool for p-type (e.g., boronfrom diborane gas) and a second complete tool for n-type (e.g.,phosphorus from phosphene gas) dopants. This requirement not onlydoubles the customer's capital equipment costs, but substantiallyincreases the risk of reduced product yield, since moving the substratesbetween systems requires further wafer handling steps and increasedexposure of the substrates to atmosphere.

Thus, the prior art bucket source technology suffers from the followinglimitations:

-   -   (1) Large footprint (width, height and length).    -   (2) High degree of expense and complexity.    -   (3) Low ion production efficiency due to the loss of B (from        B₂H₆ feed gas) and P (from PH₃ feed gas) to the walls of the ion        source due to the very large wall surface area and large volume        of the source.    -   (4) Contamination and particulate problems associated with the        rapid accumulation of deposits within the ion source associated        with (3), reducing product yield.    -   (5) Production of many unwanted ions which are implanted into        the substrate, resulting in a lack of implantation process        control and a concomitant degradation of device characteristics.        For example, significant fractions of H⁺ and BH_(x) ⁺, as well        as B₂H_(x) ⁺, are produced in the B₂H₆ plasma commonly used to        implant boron, a p-type dopant.    -   (6) Implantation of large currents of H⁺ (a result of (5))        during the implantation process limits attainable dose rate and        hence throughput, since the total ion current delivered to the        substrate must be held below a certain limit to prevent        overheating of the substrate.

SUMMARY

In one aspect, the invention provides a method of ion implantation byproducing a high brightness ion beam that extends along an axis byionizing molecules of a gas or vapor, the molecules containing animplantable species. The method includes providing an ionization chamberhaving a restricted outlet aperture; providing in the ionization chamberthe gas or vapor at a pressure substantially higher than the pressurewithin an extraction region into which the ions are to be extractedexternal to the ionization chamber, by direct electron impact ionizationby primary electrons, ionizing the gas or vapor in a region adjacent theoutlet aperture of the ionization chamber in a manner to produce ionsfrom the molecules of the gas or vapor to a density of at least about10¹⁰ cm⁻³ at the aperture while maintaining conditions that limit thetransverse kinetic energy of the ions to less than about 0.7 eV, thewidth of the ionization volume adjacent the aperture, in which saiddensity of ions is formed, being limited to a width less than aboutthree times the corresponding width of the outlet aperture; andconditions within the ionization chamber being maintained to preventformation of an arc discharge, by an extraction system, extracting ionsformed within the ionization chamber via the outlet aperture into theextraction region downstream of the aperture, thereafter, with ion beamoptics, transporting the beam to a target surface, and implanting theions of the transported ion beam into the target.

Variations of this aspect of the invention may include one or more ofthe following features. Conditions are maintained within the ionizationchamber to prevent formation of a plasma. The brightness of the ion beamupon extraction is more than about 1 mA−cm⁻²−deg⁻²×(E₀/E), where E isthe beam energy, and E₀=10 keV. The x-emittance of the beam uponextraction is less than about 70 mm-mrad×(E₀/E)^(1/2) (where E is thebeam energy and E₀=10 keV), for an ion current density of at least 1mA/cm², even for an ion mass of 120 amu. The beam noise of the stream ofions extracted through the outlet aperture is maintained below 1%. Thefield strength of any magnetic field present in the ionization chamberis less than 70 gauss. The field strength of any magnetic field presentin the ionization chamber is less than 20 gauss. There is substantiallyno magnetic field present in the ionization chamber. Any magnetic fieldpresent in the extraction region has a field strength of less than about20 gauss. The consumption of the gas or vapor is maintained less than 10sccm. The primary electrons are introduced into the ionization chamberby electron optics in a directed beam generated external to theionization chamber. The molecules to be ionized respectively comprise orconsist of at least two atoms of the implantable species.

In another aspect, the invention provides a method of ion implantationincluding producing a high brightness ion beam that extends along anaxis by providing an ionization chamber having an outlet aperture,providing in the ion chamber molecules of a gas or vapor, in which eachmolecule to be ionized comprises or consists of at least two atoms of animplantable species, ionizing the molecules and extracting ions formedfrom said molecules under conditions to produce a beam having, uponextraction, a brightness of upon extraction is more than about 1mA−cm⁻²−deg⁻²×(E₀/E), where E is the beam energy, and E₀=10 keV and anx-emittance of less than about 70 mm-mrad×(E₀/E)^(1/2) (where E is thebeam energy and E₀=10 keV), for an ion current density of at least 1mA/cm², even for an ion mass of 120 amu, thereafter, with ion beamoptics, transporting the beam to a target surface, and implanting theions of the transported ion beam into the target.

Variations of this or any other aspect of the invention may include oneor more of the following features. The molecules are dimers. Themolecules comprise decaborane. The method is conducted in a manner tocause the high brightness ion beam to have a low angular divergence atcontact with the target of less than about one degree relative to theaxis. The step of implanting the ions of the transported ion beam intothe target is employed to cause the high brightness, low divergence beamto form a drain extension region of a transistor structure on thetarget, in which the transistor structure comprises a source, a gate anda drain. The target further comprises a well dopant and the gate of thetransistor structure has a gatelength of 0.20 um or less, the drainextension intersecting the gate at a lateral junction edge, the drainextension having a lateral abruptness of 3 nm/decade or less, whereinlateral abruptness is defined as the lateral extent required toaccomplish a one decade change in the volume concentration of theimplanted species at the lateral junction edge, the junction edge beingdefined as the region where the volume concentrations of the implantedions and the well dopant are equal. The drain extension has a lateralabruptness of 2 nm/decade or less. The ions of the high brightness, lowangular divergence beam are implanted at opposite ends of the gate,sharply defining a channel beneath the gate. Sharply defining thechannel beneath the gate includes sharply defining the length of thechannel.

In another aspect, the invention provides an ion implantation system forimplanting at a desired low implant energy into a target substratecomprising an ion source for producing molecular ions (based on amolecule having a cluster of atoms of the desired species to beimplanted), an acceleration stage enabling the ions to be accelerated toa transport energy substantially greater than the desired implantenergy, and prior to the target substrate, a deceleration stage forlowering the energy of the ions to the desired implant energy.

Variations of this aspect of the invention may include one or more ofthe following features. The ion source includes an electron gun forproducing a beam of electrons at controlled energy adapted to ionize themolecules by direct electron impact ionization. The energy of theelectrons is between about 20 eV and 500 eV. The gun is arrangedrelative to an ion chamber to cause the beam of electrons to transit thechamber to a beam dump. A lengthy elongated ionization chamber has acorrespondingly elongated slot-form extraction aperture, and electronoptics following the aperture are constructed to reduce the length ofthe profile of the resultant beam, relative to the corresponding lengthof the extraction aperture, prior to the beam entering the analyzer. Theelectron optics comprises a telescope. The extraction aperture of theionization chamber is of the order of about 6 inches in length. The ionimplantation system is constructed for batch operation, a set of wafersbeing mounted on a carrier that moves relative to the beam to effectscanning. The ion implantation system is constructed as a serial ionimplanter. The ion source has a vaporizer mounted integrally with anionization chamber of the ion source, and temperature control system forthe temperature of the vaporizer. The ionization chamber electron gunand a beam dump to which the beam of electrons is directed are eachthermally isolated from the ionization chamber. The ion source is adecaborane source and the electron given is constructed to produce abeam of electron energy between about 50 and 1000 eV. The ion source isa source of As₂ ⁺ ions. The ion source is a source of P₂ ⁺ ions. The ionsource is a source of B₂ ⁺ ions. The ion source is a source of In₂ ⁺ions. The ion source is a source of Sb₂ ⁺ ions.

In another aspect, the invention provides a method of conducting ionimplantation by use of the ion implantation systems of any of the otheraspects of the invention.

In another aspect, the invention provides a method of ion implantationof ions at a desired implant energy on a target substrate. The methodincluding forming molecular ions (based on a molecule having a clusterof atoms of the desired species to be implanted, accelerating the ionsto and transporting the ions at an energy substantially above theimplant energy, and prior to implant upon the substrate, deceleratingthe ions to the implant energy.

Variations of this aspect of the invention may include one or more ofthe following features. The ions are decaborane. The ions are P₂ ⁺ ions.The ions are B₂ ⁺ ions. The ions are In₂ ⁺ ions. The ions are Sb₂ ⁺.

In another aspect, the invention provides an ion implantation systemincluding an ion implanter having an ion extraction system; an ionsource capable of providing ions in commercial ion current levels to theion extraction system, the ion source including an ionization chamberdefined by walls enclosing an ionization volume, one of said wallsdefining an extraction aperture of a length and width sized and arrangedto enable the ion current to be extracted from said ionization volume bysaid extraction system; an electron gun constructed, sized and arrangedwith respect to the ionization chamber to project a directional beam ofprimary electrons along an axis through the ionization chamber; and abeam dump aligned with the electron gun to receive the directional beam,the beam dump being maintained at a substantial positive voltagerelative to the emitter voltage of the electron beam-gun, the axis ofthe beam path of said primary electrons extending in a directiongenerally adjacent to the aperture, the electron beam having a dimensionin the direction corresponding to the direction of the width of theextraction aperture that is about the same as or larger than the widthof the aperture.

Variations of this aspect of the invention may include one or more ofthe following features. The ion implantation system further includes avaporizer arranged to introduce vapor to the ionization volume. The ionimplantation system further includes a gas passage for introducing gasfrom a gas source to the ionization volume. The ion implantation systemfurther includes a control system enabling control of an energyassociated with the primary electrons to ionize individual vapor or gasmolecules principally by collisions with primary electrons from theelectron gun. The vapor comprises decaborane. The directional beam is aribbon ion beam. The ribbon beam is of shorter extent than the length ofthe ion extraction aperture. The ribbon beam is longer than the ionextraction aperture. The ribbon beam is about the same length as thelength of the ion extraction aperture. The length of the aperture is atleast as long as the length or width of a target substrate.

In another aspect, the invention provides a method of irradiating anextended panel of predetermined dimensions, the method comprisinggenerating a ribbon ion beam with the ion implantation system of any ofthe other aspects of the invention, and directing the ribbon ion beamonto a surface of the extended panel.

Variations of this aspect of the invention may include one or more ofthe following features. The extended panel is a flat panel, the methodincluding irradiating the flat panel across substantially an entirepanel surface. The ribbon ion beam produced is stationary, the flatpanel being mechanically scanned across the beam to accomplish iondoping of the panel. The length of the ribbon beam is longer than theflat panel substrate dimension orthogonal to the scan direction.

Electron-Beam Ion Source for Ion Implantation Advantages

In one aspect, the present invention reduces or eliminates theabove-described problems associated with conventional ion implantationsources. The disclosed solution has the following features which resultin an exceptionally low-emittance ion source, ideally suited to therequirements of next-generation ion implanters:

-   -   1) There is no plasma, hence no plasma potential.    -   2) The ion density is low (on the order of 10¹¹ cm⁻² or less),        reducing coulomb scattering between ions, and the resultant ion        energy spread, to a negligible degree.    -   3) Gas molecules are ionized by direct electron impact        ionization, resulting in “cold” ions which possess thermal        energies approximately equal to that of the neutral gas        molecules, that is, <<1 eV. This results in a highly        monochromatic source of ions, and enables low angular divergence        in the extracted ion beam.    -   4) By tuning the electron impact energy, a high percentage of        the dopant ions of interest can be produced, minimizing        space-charge effects.    -   5) Molecular ions, which are typically dissociated in an arc        discharge, are preserved to a high degree. For example, when        using phosphene (PH₃) feed gas, a large percentage of the PH₃ ⁺        ion can be produced (e.g., 50% of total extracted current). As        another example, decaborane (B₁₀H₁₄) can be used to produce a        large fraction (>70%) of the decaborane ion, B₁₀H_(x) ⁺. This        ion is very important for implanting boron at very low (<1 keV)        energies, and its use can greatly increase the implanted boron        dose rate. Plasma-based ion sources such as the Enhanced Bernas        source cannot produce decaborane ions since plasma effects and        elevated wall temperature cause dissociation and subsequent loss        of the decaborane molecule.    -   6) A magnetic field is not required.    -   7) The high-frequency noise observed in arc discharge sources is        absent, preserving to a much higher degree space charge        compensation by low-energy electrons in the beam plasma.    -   8) The dimension of the ion extraction aperture is scalable over        a broad range, from 12 mm to 300 mm or greater, for example.        This leads to greater extracted currents, and improved        compatibility with next-generation ion implanter designs. In        fact, this feature enables ion implanter designs not possible        with previous ion source designs.        Acceleration/Deceleration Ion Implantation Advantages

According to one aspect of the invention, I provide a technique forobtaining a great increase in low-energy beam current and higher qualityand higher production rates for low energy ion implantation, by novelcombination of acceleration/deceleration ion implantation with molecularions as the species being implanted. By this combination, I realizeconditions that alleviate the aforementioned non-uniformity anddosimetry problems that have been seen as being inherent inacceleration/deceleration ion implantation systems.

In molecular ion implantation in such systems, an ion beam is formedfrom a compound which contains multiple atoms of the dopant of interest(for example, B, As, P, Sb, or In) to produce both a shallowerimplantation profile and a higher effective dose rate than possible withconventional monomer (i.e., single atom) ion implantation. As animportant example applied to low-energy boron implantation, rather thanimplanting an ion current I of monomer B⁺ ions at an energy E, adecaborane molecular ion, B₁₀H_(x) ⁺, is implanted at an energy 10×E andan ion current of 0.10×I. Extending this example, a 5 keV, 1 mA B₁₀H_(x)⁺ ion beam is process-equivalent to a 500 eV, 10 mA B⁺ ion beam. Theresulting implantation depth and dopant concentration (dose) of thesetwo methods have been shown to be equivalent, but the decaboraneimplantation technique has significant advantages. Since the transportenergy (mass×velocity²) of the decaborane ion is ten times that of thedose-equivalent boron ion, and the ion current is one-tenth that of theboron current, the space charge forces responsible for beam blowup andthe resulting beam loss are much reduced relative to monatomic boronimplantation. As mentioned above, this approach has been proposed toincrease useful boron dose rates of conventional (i.e. non-deceleration)ion implanters. In one aspect, my contribution is the specialized use ofmolecular (cluster) ions in acceleration/deceleration ion implantationto realize unexpected advantages.

According to a preferred embodiment of the present invention, decaboraneions are employed in an acceleration/deceleration ion implanter, gaininga large increase in useful boron dose rate, and, heretoforeunappreciated by those of ordinary skill, avoiding the substantialdegradation of beam profile characteristics at the wafer that areinherent in decelerating monomer boron ions prior to implantation. Alsoaccording to the invention, other important molecular dopants canachieve similar effects. An explanation of the improvement is based onthe following considerations.

It is well-known that space-charge effects impose limits on attainablebeam currents in the initial ion extraction stage of a conventional(i.e., non deceleration) ion beam implanter as well as in the beamtransport stage. Considering the ion extraction stage, the relativeimprovements enabled by molecular implantation can be quantified byinvestigating the Child-Langmuir limit, that is, the maximum spacecharge-limited ion current density which can be utilized by theextraction optics of the ion implanter. Although this limit dependssomewhat on the design of the implanter optics, it has been recognizedto be usefully approximated as follows:J _(max)=1.72(Q/A)^(1/2) U ^(3/2) d ⁻²,  (1)where J_(max) is in mA/cm², Q is the ion charge state, A is the ion massin amu, U is the extraction voltage in kV, and d is the gap width in cm.In practice, the electrostatic extraction optics used by many ionimplanters can be made to approach this limit. By extension of equation(1), the following figure of merit, Δ, which quantifies the easing ofspace-charge limitations in the case of molecular implantation relativeto monatomic implantation, can be expressed as:Δ=n(U _(n) /U _(l))^(3/2)(m _(n) /m _(l))^(−1/2),  (2)where Δ is the relative improvement in dose rate (atoms/s) achieved byimplanting a molecular compound of mass m_(n) and containing n atoms ofthe dopant of interest at an accelerating potential U_(n), relative to amonatomic implant of an atom of mass m_(l) at an accelerating potentialU_(l). In the case where U_(l) is adjusted to give the same implantationdepth into the substrate as the monomer implant, equation (2) reducesto:Δ=n².  (3)

Thus, up to a factor of 100 increase in dose rate can be accomplished bysubstituting decaborane for boron at ion extraction in a conventional(non deceleration) system.

I realize that the deceleration stage of an acceleration/decelerationimplantation system has a similarity to the operation of extractionoptics of an extraction stage that is crucially relevant to the issue;both employ a strong focusing field over a short distance. I realizethat equation (1) has a sufficient degree of validity for thedeceleration stage to enable comparison of its performance withmolecular ions and with monomer ions. Hence, I realize equation (3) canalso be used to evaluate the deceleration stage. Using this mode ofanalysis, for example, a conventional acceleration/decelerationimplanter can deliver up to about 2 mA of boron monomer to the wafer atan implantation energy of 500 eV, albeit with the significantnon-uniformity and dosimetry problems mentioned; but by the novelsubstitution of decaborane (B₁₀H_(x) ⁺) for boron monomer in theacceleration/deceleration ion implanter, made possible in a productionworthy system by using the techniques described in my above referencedpatent applications the same dose rate can be accomplished by implanting0.2 mA of decaborane at 5 keV. This reduces sensitivity to space-chargeto such an extent in the deceleration stage that the usual degradationof beam profile that occurs with deceleration, and implant uniformity,angular integrity, and dosimetry of an acceleration/deceleration systemare greatly improved.

This novel accel/molecular ion combination (acceleration/decelerationion implantation, using a beam of molecular (cluster) ions), can beemployed to increase the low-energy boron dose rate to new proportions,never before possible in ion implantation. For example, one can envisionextracting more than 3 mA of decaborane at 20 keV, and decelerating thedecaborane ions down to 5 keV (a 4:1 deceleration) to achieve a doserate of up to 30 mA at an effective implantation energy of 500 eV! Sucha large effective boron dose rate easily enables high dose implants suchas PMOS source/drain extensions at a mechanical throughput limit inexcess of 200 wafers per hour (200 WPH), even for 300 mm-diametersubstrates (for reference, 2 mA of conventional boron will produce awafer throughput of about 25 WPH at a dose of 8E14). As I will discusslater, such high beam currents will also enable novel, importantprocesses for ultra-shallow junction formation.

Such an acceleration/deceleration system can also be used for dimerimplantation. Ion beams consisting of dimers (typically not heretoforerecognized as suitable ion implantation materials), can be utilized toreap above-described benefits with other dopant species, using theproduction-worthy vaporization and ionization techniques provided in myabove-referenced patent application. Ion beams of, for example, As₂ ⁺,P₂ ⁺, B₂ ⁺, In₂ ⁺, or Sb₂ ⁺ can be formed, and according to myrealization of the beneficial applicability of equation (3) to thedeceleration stage, can yield a factor of 4 improvement in thedecelerated beams, increasing maximum dose rate and reducingnon-uniformity and dosimetry problems in the manner previously describedfor decaborane implantation. Table Ia below lists materials suitable fordimer implantation as applied to the present invention.

TABLE Ia Melting Pt (deg Compound C.) Dopant Phase As₂O₃ 315 As₂ SolidP₂O₅ 340 P₂ Solid B₂H₆ — B₂ Gas In₂(SO₄)₃XH₂O 250 In₂ Solid Sb₂O₅ 380Sb₂ Solid

As part of the system and method, such dimer compounds are vaporized attemperatures below their melting points, and the vapor is ionizedprincipally by impact action of a broad electron beam transiting avolume containing the vapor.

The use of the disclosed systems for acceleration/decelerationimplantation of decaborane, etc. enables new processes in semiconductormanufacturing. Another aspect of the invention is the realization thatone or more costly steps can be eliminated from many implant sequences,or their cost reduced, or the quality of the implant sequence improved,by using in the sequence, the combination decel/molecular ion method andsystem described above.

For example, such a system can be used for the amelioration of transientenhanced diffusion (TED). In the creation of ultra-shallow p-n junctionsin CMOS manufacturing, special attention is given to forming PMOSsource/drain (S/D) structures. Boron is the only p-type dopant having ahigh enough solid solubility to form S/D structures with the requiredelectrical conductivity; however boron will diffuse rapidly in thesilicon substrate during the anneal (“activation”) cycle that isrequired to process the wafers. This anomalous boron diffusion, calledtransient enhanced diffusion (TED), limits the attainable parameters, inparticular the abruptness of the p-n junction. TED is believed to bemediated (detrimentally increased) by defects created in the siliconduring the implantation process.

In forming leading-edge, ultra-shallow semiconductor chip devices,manufacturers wish to use very low-energy (sub-keV) boron implants toform very shallow as-implanted boron profiles, so that the activatedprofile is largely determined by TED. In order to reduce the extent ofTED, a low thermal budget spike anneal (i.e., rapid thermal annealing orRTA) is being used in conjunction with sub-keV implants to achieve moreshallow p-n junctions. Recently, it has been proposed that a boronimplantation energy of 500 eV is probably the lowest energy boronimplant employable in order to minimize the depth of the activated p-njunction, since TED is expected to dominate the profile at this andlower energies after activation. However, I regard this conjecture to befar from proven in manufacturing, since the effects of TED reducesomewhat linearly as the boron implantation depth is decreased. Thisadvantageous “shallowness” effect upon TED is believed to stem from thefact that the exposed silicon surface acts as a “sink”, or getter, fordefects during TED, so that the shallower the implant, the less theextent of TED.

Since wafer throughput unfortunately is already far below mechanicallimit when performing 500 eV boron implants, even in ion implantersusing deceleration, and since reducing the implantation energy below 500eV causes the wafer throughput to drop further dramatically, suchsub-500 eV implants using conventional boron implantation are unlikelyto be used in manufacturing. Such effects of reduced throughput are, ofcourse, much more harmful economically for the 300 mm manufacturing thatis requiring great capital cost for new fabs and equipment.

However, by employing the new system and method provided here,commercially advantageous throughput of ultra shallow implants can beachieved, with “shallowness” amelioration of the TED problem, and thusachievable density and performance of implanted devices can be extendedto new regimes of quality and smallness of size.

The disclosed acceleration/deceleration systems can also reduce the needto pre-amorphicize. To assist in limiting the depth of the as-implantedboron profile, pre-amorphization (destruction of the crystal lattice)implants have often been performed in advance of the boron implant tolimit channeling, and thus increase the as-implanted depth profile.Amorphization is accomplished by implants of high doses of germanium orsilicon beams. This is an expensive added process which increases thecost and complexity of manufacturing ultra-shallow p-n junctions.

I conceive that new process advantages can be obtained in this respectas well by special use of the cluster molecule dopants in anacceleration/deceleration machine in the manner described. That is tosay, not only can improved boron dose rate, shallower implants andimproved device performance be achieved, but also the damagecharacteristics of this new molecular implantation system makes possibleelimination of the expensive Ge or Si pre-amorphization implant steps.It is known that high density ion clusters such as decaborane causedamage to the local crystal structure upon impact with the siliconsurface due to the inelastic nature of the collision. At sufficientlyhigh dose rates (achievable with the present invention, e.g., between0.5 and 3 mA of decaborane), the resulting damage profile can obviate orreduce the need for a separate pre-amorphization implant, eliminating orreducing the cost of this expensive step in the manufacturing process.

Thus, boron implants at production-worthy wafer throughputs withacceleration/deceleration systems employing decaborane ions can beperformed for both 200 mm and 300 mm substrates at energies as low as100 eV. Since TED effects will be further reduced at these extremely lowimplantation energies, shallower p-n junctions can be fabricated thanheretofore possible.

By ionizing the molecule to produce the molecular ions by primaryelectron-impact, heat sensitive ion source materials can be employed,especially solid decaborane and the dimers mentioned above.

By employing a broad beam of electrons directed adjacent to a greatlyelongated extraction aperture, and employing telescopic optics to reducethe dimension of the beam prior to the beam entering the analyzer of thebeam line.

The invention makes possible:

1) Production-worthy wafer throughput for boron implants usingdecaborane ions at implantation energies between 100 eV and 1 keV, for200 mm and 300 mm wafers;

2) By use of a high dose rate of decaborane (e.g., between about 2×10¹⁵and 2×10¹⁶ decaborane ions per second), producing enough crystalstructural damage to obviate or reduce the need for costlypre-amorphization implants;

3) By using extremely low implantation energy (between about 1 keV and 5keV decaborane energy, equivalent to between 100 eV and 500 eV boronenergy) ultra-shallow junctions by a reduction in the broadening of theactivated boron profile due to TED.

4) By use of other cluster molecules, including novel dimer materials,achieving similar advantageous results with other implant species.

Thus, fewer steps, significant cost reduction, shallower and more densep-n junctions, and improved device performance can be achieved thanheretofore possible.

Electron-Beam Ion Source for Ion Doping Advantages

In another aspect, the present invention can be implemented into an iondoping system as a replacement for the above-described bucket source.The disclosed ion doping system offers the following advantages:

-   -   (1) Small footprint—the electron-beam ion source is a        small-volume source, and is elongated in only one direction, the        desired length of the ribbon beam.    -   (2) Reduced cost—because of its compact size and scalability,        the present invention is simpler and significantly less        expensive than the prior art.    -   (3) High efficiency—because of its smaller volume and reduced        surface area, loss of the ions of interest to the walls of the        ionization volume is substantially reduced relative to the prior        art.    -   (4) Improved process control—a higher percentage of ions        produced are the desired ions. This leads to reduced deposition        (fewer ions are produced in the first place to achieve a given        ion beam current), higher ion production efficiency of the        dopant ion of interest, and much better control of the        implantation process. Since the majority of ions produced from        the electron-beam ion source are the ions of interest, the        implantation profile and dose accuracy is much improved relative        to the prior art.    -   (5) Higher throughput—the present invention increases throughput        due to its ability to produce higher dopant ion currents than        the prior art.    -   (6) Soft ionization—the present invention enables the efficient        production of molecular ions such as decaborane, which offer        significant advantages in throughput and efficiency in ion        doping applications over, for example, diborane.

Reduced capital equipment cost, less fab floor space occupied, andhigher product yield—due to its small footprint and reduced complexity,the present invention also enables a single ion doping system to beconstructed with two ion sources, one for p-type dopants, and one forn-type dopants. Using simple dual slit optics, the ion doping system canswitch between the two ion sources while processing a lot of substrates.This relieves the high equipment cost of two dedicated systems, halvescostly fab floor space, and reduces the risk to product yield which ispresently a consequence of prior art ion doping systems.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a prior art source for ion implantation.

FIG. 2 is an enlarged schematic view of a portion of the prior artsource for ion implantation of FIG. 1.

FIG. 3 is a diagrammatic view of an ion implantation source of thepresent invention, shown in a cutaway view down the center axis of thesource, so the internal components can be seen.

FIG. 4 is a magnified diagrammatic view of the ionization chamber of theion implantation source of FIG. 3.

FIG. 4 a is a diagrammatic view of FIG. 4 a of a preferred embodiment ofthe electron optics of the ion implantation source of FIG. 3.

FIG. 5 is a schematic diagram of the biasing scheme of power supplieswhich provide voltage to the ion implantation source of FIG. 3.

FIGS. 6 a and 6 b are diagrammatic cross-sectional side and top views,respectively, of an alternate ionization chamber for an implantationsource of the present invention.

FIGS. 7 a and 7 b are diagrammatic perspective and top views of anapparatus for improving the focus of the electron beam of the ionizationchamber of FIG. 6.

FIG. 7 c is a schematic illustration of dimensional geometry of theapparatus FIGS. 7 a and 7 b.

FIG. 8 is a diagrammatic view of the apparatus of FIGS. 7 a and 7 bincorporated into an ion implantation source similar to that depicted inFIG. 3.

FIG. 9 is a diagrammatic top view of an alternative ionization chamberof the present invention.

FIG. 10 is a diagrammatic view similar to that of FIG. 3 illustratingthe ionization chamber of FIG. 9 incorporated into an ion implantationsource.

FIG. 11 is a general schematic illustration of ion implantation.

FIG. 12 is a general schematic illustration of an ion source emitting anion beam.

FIG. 13 is a general schematic illustration of ion implantation forminga drain extension adjacent a gate on a target substrate.

FIG. 14 a is a general schematic illustration of a gate edge and drainextension layer formed by boron ion implantation of a previouslyphosphorus doped silicon substrate while FIG. 14 b is a graphicalillustration of boron and phosphorus ion concentrations taken alongsection AA of FIG. 14 a and charted on a logarithmic scale.

FIG. 15 is a graphical illustration of the lateral straggle exhibitedduring ion implantation as a function of the incident angle of ionapproach to the target and the ion implantation energy.

FIGS. 16 a and 16 b are schematic illustrations of predicted lateralstraggle exhibited during ion implantation for normal incidence and 7degree nonparallel incidence angles, respectively.

FIG. 17 is a view similar to that of FIG. 4, illustrating an alternativeion implantation source of the present invention for producing extendedribbon beams.

FIG. 18 a is a diagrammatic view of a dual ion source system of thepresent invention for enabling both n- and p-type dopants to beimplanted in a single ion doping tool.

FIG. 18 b is a diagrammatic view of the ion source of FIG. 18 aimplanting ions onto a flat panel substrate.

FIG. 19 is a diagrammatic view of a doping tool for flat panel displaysinto which the arrangement of FIG. 18 a can be incorporated.

FIG. 19 a is a graphical illustration of a cracking pattern fordecaborane.

FIG. 20 is a side cross-sectional view of an acceleration/decelerationion implantation system, of the kind having a fixed beam line, wafersbeing carried on a spinning disk.

FIG. 21 is a side view of an ion source, suitable for decaborane, etc.,retrofittable into the ion source housing of a pre-existingacceleration/deceleration ion implantation system, such as shown in FIG.1.

FIG. 22 is a side view of another ion source suitable for decaborane,etc., used in an acceleration/deceleration ion implantation system,featuring a highly elongated extraction aperture for producing aninitial beam of ions having a highly extended cross-section;

FIG. 23 is a view, similar in kind to FIG. 2, of an ion source capableof magnetically confining an electron beam passing through theionization chamber;

FIG. 24 is a diagrammatic representation of ion optics combined with anionization chamber from which a beam of ions of highly extendedcross-section is extracted.

FIG. 25 is an unscaled perspective view of a lens and lens holdercombination of the present invention.

FIG. 26 is an unsealed cross-sectional perspective view of the electrongun of FIG. 27.

FIG. 27 is an unsealed perspective view of an electron gun of thepresent invention.

DETAILED DESCRIPTION

The following terms and definitions apply throughout the application.

Transverse kinetic energy (E_(T)): The component of kinetic energytransverse to the direction of beam propagation, i.e., the direction ofthe extraction field. E_(T)=½mv_(T) ², where v_(T) is the component ofvelocity orthogonal to the beam direction.

Beam noise (N): Fluctuation in beam current intensity as a percentage ofaverage current level, above a frequency of 100 Hz.

Emittance (ε): The total emittance ε is the product of the twoemittances, ε=ε_(x)ε_(y), where in the case of vertically-oriented slotlenses, ε_(x) is the emittance in the horizontal direction (along theslot width), and ε_(y) is the emittance in the vertical direction. Forany lens geometry, ε_(x) and ε_(y) are defined along the two orthogonaldirections normal to the direction of beam propagation. The emittancecomponents e_(i) are defined as follows:ε_(x)=2κΔxα_(x), ε_(y)=2κΔyα_(y),whereκ=(E₀/E)^(1/2),where E is the beam energy and E₀=10 keV;

α_(x) and α_(y) are the beam divergence half-angles into the x- andy-directions, respectively; and Δx and Δy are the be am dimensions inthe x- and y-directions, respectively, and the emittance variables areall measured at the same z-position along the direction of propagation,and are chosen to contain at least 70% of the beam current. Emittancecomponents e_(i) are expressed in units of mm-mrad or in cm-deg.

Brightness (B): B is the beam current I divided by total beam emittance:B×I/ε_(x)ε_(y).

Plasma is defined as a region containing the ionization volume which issubstantially electrically neutral, containing electrons and ionscontributing approximately equal charge densities opposite in sign.

Ion Implantation Sources

Referring now to the attached figures, a conventional ion source used inion implantation is shown in FIGS. 1 and 2. The Enhanced Bernas sourceis commonly used in high current, high energy, and medium current ionimplanters. The ion source a is mounted to the vacuum system of the ionimplanter through a mounting flange b which also accommodates vacuumfeedthroughs for cooling water, thermocouples, dopant gas feed, N₂cooling gas, and power. The dopant gas feed c feeds gas into the arcchamber d in which the gas is ionized. Also provided are dual vaporizerovens e, f in which solid feed materials such as As, Sb₂O₃, and P may bevaporized. The ovens, gas feed, and cooling lines are contained within acooled machined aluminum block g. The water cooling is required to limitthe temperature excursion of the aluminum block g while the vaporizers,which operate between 100 C and 800 C, are active, and also tocounteract radiative heating by the arc chamber d when the source isactive. The arc chamber d is mounted to, but in poor thermal contactwith, the aluminum block g. The ion source a is an arc discharge source,which means that it operates by sustaining a continuous arc dischargebetween an immersed hot-filament cathode h and the internal walls of thearc chamber d. Since this arc can typically dissipate in excess of 300W, and since the arc chamber d cools only through radiation, the arcchamber can reach a temperature in excess of 800 C during operation.

The gas introduced to arc chamber d is ionized through electron impactwith the electron current, or arc, discharged between the cathode h andthe arc chamber d. To increase ionization efficiency, a uniform magneticfield i is established along the axis joining the cathode h and ananticathode j by external magnet coils 90, shown in FIG. 2, to provideconfinement of the arc electrons. An anticathode or repeller electrode j(located within the arc chamber d but at the end opposite the cathode h)is typically held at the same electric potential as the cathode h, andserves to reflect the arc electrons confined by the magnetic field iback toward the cathode h and back again repeatedly. The trajectory ofthe thus-confined electrons is helical, resulting in a cylindricalplasma column between the cathode h and anticathode j. FIG. 2 shows apossible electron trajectory between cathode and anticathode, which ishelical due to the confining magnetic field B. The plasma density withinthe plasma column is typically high, on the order of 10¹² per cubiccentimeter; this enables further ionizations of the neutral and ionizedcomponents within the plasma column by charge-exchange interactions, andalso allows for the production of a high current density of extractedions. The cathode h is typically a hot filament or indirectly-heatedcathode, which thermionically emits electrons when heated by an externalpower supply. It and the anticathode are typically held at a voltageV_(c) between 60V and 150V below the potential of the ionization chamberd. High discharge currents D can be obtained by this approach, up to 10A. Once an arc discharge plasma is initiated, the plasma develops asheath adjacent to the surface of the cathode h (since the cathode h isimmersed within the arc chamber and is thus in contact with theresulting plasma). This sheath provides a high electric field toefficiently extract the thermionic electron current for the arc; highdischarge currents can be obtained by this method.

The discharge power P dissipated in the arc chamber is P=DV_(c), orhundreds of watts. In addition to the heat dissipated by the arc, thehot cathode ht also radiates power to the arc chamber d walls. Thus, thearc chamber d provides a high temperature environment for the dopantplasma, which also boosts ionization efficiency relative to a coldenvironment by increasing the gas pressure within the arc chamber d, andby reducing condensation of dopant material on the hot chamber walls.

If the solid source vaporizer ovens e or f are used, the vaporizedmaterial feeds into the arc chamber d through vaporizer feeds k and l,and into plenums m and n. The plenums serve to diffuse the vaporizedmaterial into the arc chamber d, and are at about the same temperatureas the arc chamber d. Radiative thermal loading of the vaporizers by thearc chamber also typically prevents the vaporizers from providing astable temperature environment for the solid feed materials containedtherein below about 100 C. Thus, only solid dopant feed materials thatboth vaporize at temperatures >100 C and decompose at temperatures >800C (the nominal wall temperature of a Bernas source) can be vaporized andintroduced by this method.

FIG. 3 shows one embodiment of the present invention, certain aspects ofwhich are also described in the above-referenced patent applications,shown in a cutaway view down the center axis of the source, so theinternal components can be seen. The external vaporizer 28 is comprisedof vaporizer body 30 and crucible 31 in which solid source feed material29 such as decaborane resides. Resistive heaters are imbedded into thevaporizer body 30, and water cooling channels 26 and convective gascooling channels 27 are in intimate contact with vaporizer body 30, andare used in combination to provide a uniform operating temperature aboveroom temperature to the crucible 31. Thermal conduction between thecrucible 31 and the temperature-controlled vaporizer body 30 is providedby pressurized gas introduced by a gas feed 41 into thecrucible-vaporizer body interface 34, while the temperature of thevaporizer housing is monitored through a thermocouple. Vaporizeddecaborane or other vaporized material 50 is fed through vaporizer exitchannel 39 and through heated gate valves 100 and 110 into theionization chamber 44 through conductance channel 32. The sourcemounting flange 36 and source block 35 are also temperature controlledto a temperature near or above the vaporizer temperature.

The ion source gas delivery system includes two conduits that feed theionization chamber from two separate sources. The first is a smalldiameter, low-conductance path from a pressurized gas source such as agas cylinder. The second is from a high-conductance path from alow-temperature vaporizer, which vaporizes solid material. Regardless ofthe source, the gas delivery system maintains a gas concentration of afew millitorr in the ionization chamber. The vaporizer maintains tighttemperature control of its surfaces that contact the solid material, inorder to maintain stable concentrations of gas in the ionizationchamber.

Referring again to FIG. 3, the vaporizer assembly, 30 a, is comprised ofa heated and cooled body, 30 and a removable crucible, 34. Access to thecrucible is made possible by removing the end plate, 28 on the back ofthe vaporizer.

Once the crucible is removed from the vaporizer, it can be recharged byremoving its cover, 34 b that is elastomerically sealed to the end ofthe crucible and raising the grate, 34 a, which contains the solid, 29.

After recharge the crucible is inserted in the body and a gas seal ismade to the bore, 39, at the front end of the body. This bore, 39 is theexit for the vaporized gas. The mechanical fit between the crucible andthe body is precisely maintained to achieve temperature uniformity ofthe crucible. The gap is filled with a gas (cool gas) to facilitatethermal transfer between the two surfaces. The cool gas enters the gapthrough an end plate fitting, 28 a.

Temperature control is performed using PID closed loop control ofresistive elements that are imbedded in the body. The body material ishighly thermally conductive to maintain temperature uniformity. A smallthermal leak is intentionally applied to the body to create stability inthe control system using external air channels. The air channels 27 passaround the vaporizer body and are covered by plates that are not shown.Air is ducted to the channels within a manifold system, which isintegrated into the vaporizer end plate, 28, to provide slightconvective cooling. The air is fed through the inlet after proceedingpast a metering valve used for flow control. The air discharges from theassembly into house exhaust.

In addition to the air-cooling, there are also provisions for liquidcooling the vaporizer body. Coolant is ducted through a 1-meter long 6mm diameter bore that travels back and forth throughout the body.Connections are made through fittings mounted to body ports, 26. Theliquid cooling provides rapid cooling of the vaporizer assembly toprovide quick service turnaround and also to change solid species.

Referring now to FIG. 4, ionization chamber 44 is in good thermalcontact with block 35 through pressurized gas conducted through aconduit into the interface 36 between ionization chamber 44 and block35. Gaseous materials, for example process gases such as PH₃ which areheld in gas cylinders, can be fed into the ionization chamber 44 throughgas feed 33. Typically, the gas pressure within the ionization chamber44 is approximately 1×10⁻³ Torr, while the region external to theionization chamber 44 is approximately 1×10⁻⁵ Torr. Referring now toFIG. 4, the electron beam 125 produced by electron gun 42 enters theionization chamber 44 through electron beam entrance aperture 45, andtransits the ionization chamber 44 parallel to and in close proximity tothe ion extraction aperture slot 46 contained within ion extractionaperture plate 80, exiting the ionization chamber 44 through electronbeam exit aperture 47 and intercepted by beam dump 70. Beam dump 70 iswater cooled through water cooled holder 130, which carries electricallyresistive (>10 MΩ-cm) de-ionized water. The beam dump is electricallyisolated by insulating standoff 56, so that the e-beam currentintercepted by beam dump 70 can be monitored externally at HV waterfeedthrough 170. The ion extraction aperture 80 is electrically isolatedfrom ionization chamber 44 by an electrically insulating, thermallyconductive gasket, and biased to a negative potential relative toionization chamber 44. This bias of ion extraction aperture 80establishes an electric drift field which attracts the ions towards theaperture 80, and provides a drift velocity to the otherwise thermalions, enabling higher extracted ion currents than possible in theabsence of a drift field. Typical dimensions for some of the ion sourcestructures are: a 7.5 mm diameter round electron entrance aperture 45, a10 mm diameter electron beam exit aperture 47, a 25 mm diameter by 65 mmlong electron gun assembly 42, and a 67 mm tall ionization chamber 44.Cutout 48 in the gun housing 142 enable the portion of the electron gunassembly 42 which contains the cathode to be exposed to the vacuumenvironment of the source housing, extending the service lifetime of thecathode 143.

The electron gun optics consist of the cathode 143, beam shapingelectrode 145, first anode 147, focus electrode 149, second anode 150,and exit lens 152. This lens system extracts a space charge-limitedelectron current, and the downstream four-element lens comprised offirst anode 147, focus electrode 149, second anode 150, and exit lens152 both collimates and accelerates the electron beam to the desiredfinal energy. Thermionic electrons are emitted by the hot cathode 143,which may be constructed of refractory metal or LaB₆, for example, andmay be heated directly or indirectly. The electrons are acceleratedacross the first anode gap in a Pierce geometry, the equipotentialsbetween cathode 143 and first anode 147 being shaped by the conicalbeam-shaping electrode 145 and first anode, which maximizes the outputcurrent by allowing for space-charge effects. They can be extracted attip to 5 keV, and decelerated to a final energy which is variablebetween about 70 eV and 2000 eV by the downstream optics.

FIG. 4 a shows a preferred embodiment of the electron optics, in whichthe second anode 150′ and exit lens 152 are shaped according to a Piercegeometry. This Pierce geometry is inverted from the geometry of thePierce extractor 144 defined by lenses 145′ and 147′, so that the lens153 defined by the concatenation of 150′ and 152′ can be efficientlyused as a deceleration lens, introducing a low-energy (e.g., 100 eV orless), generally collimated electron beam into the ionization chamber44. The incorporation of a “reverse Pierce” geometry for thedeceleration lens 153 helps correct for a space charge-limited electronbeam, so that a higher and more collimated low energy electron flux maybe introduced into the ionization chamber 44 than otherwise possible.For example, the electron beam may be extracted into lens 147′ at 1 keV,propagate into lens 150′ at 500 eV, and decelerate to 100 eV within lens152′, enabling a higher electron current than if the beam were extractedat 100 eV at the extractor 144.

The space charge forces present within the electron gun optics andespecially within the deceleration lens 153 can be further amelioratedby the intentional production of positive ions along the electron beampath. The positive space charge of the ions compensates for the negativespace charge of the electrons, reducing the net coulomb repulsionbetween electrons within the beam, thus reducing beam blow-up andenabling higher electron currents than otherwise possible. This is aneffective means for reducing space charge forces: since the ions areheavy and slow-moving, their depletion rate is low, and a reasonablecharge balance can be maintained if the rate of ion production issimilar to the ion loss rate. The ion production rate at any point inthe electron beam path is proportional to the local pressure ofionizable gas at that point. For example, consider local gas pressuresP1>P2>P3>P4 shown in FIG. 4 a (I note that it is possible to control thelocal pressures Pi by tailoring the conductance between individual lenselements and the ambient vacuum). Further consider the predominant gasspecies being decaborane (B₁₀H₁₄), a large, heavy molecule with a largeelectron-impact ionization cross section for the production of positiveions. Within the ionization chamber 44, P1 is highest, approximately10⁻³ Torr, so that space charge compensation is very effective. Thisensures uniform propagation of the electron beam within the primaryionization region, ensuring good uniformity of the ion density adjacentto the ion extraction aperture 46, and hence a uniform ion density inthe extracted ion beam. P4 is the ambient pressure of the source vacuumhousing (nominally 10⁻⁵ Torr or less), and because the gas within theionization chamber 44 propagates through electron beam entrance aperture45′, a large pressure gradient is established between these extrema.Since the deceleration lens 153 is close to 45′, P2 is relatively high,and space charge repulsion is reduced substantially. The region P3 isadjacent to the cathode 143′, hence it is desired to maintain P3 closeto P4, i.e., at a sufficiently low pressure that the arrival rate ofdecaborane molecules to the cathode surface is less than the desorptionrate of decaborane byproducts which can deposit on the cathode surface.This is particularly important for low-temperature cathodes such asLaB₆, and for field emitter cathodes. In general, refractory metalcathodes operate at sufficiently high temperatures that deposition ofcracked process gases is not a problem.

FIG. 5 shows the biasing scheme of power supplies which provide voltageto the electron gun elements and to the ion source, and dedicatedmeters. The symbols used in FIG. 5 have the following significance:

-   -   V_(S) (source V): 0-40 kV pos @ 100 mA. Sets ion beam energy,        biases ionization chamber relative to ground.    -   V_(C) (cathode V): 0-2 kV neg @ 100 mA. Sets electron beam        energy, biases cathode relative to ionization chamber.    -   V_(F) (filament V): 0-5 V @ 50 A. Provides heater current to        directly- or indirectly-heated cathode emitter.    -   V_(E) (extraction aperture V): 0-20 V neg @ 100 mA. Biases ion        extraction aperture relative to ionization chamber.    -   V₁ (Cathode shield V): 0-50 V neg @ 10 mA. Biases beam-shaping        electrode relative to cathode.    -   V2 (anode V): 0-5 keV pos @ 50 mA. Biases first anode relative        to cathode.    -   V3 (focus V): 0-5 keV pos @ 10 mA. Biases focus electrode        relative to cathode.    -   V4 (exit lens V): 0-2 keV pos @ 50 mA. Biases exit lens relative        to cathode and determines the energy at which electron beam        leaves tetrode consisting of cathode, anode, focus, and exit        elements.    -   V_(D) (beam dump V): 0-2 kV pos @ 100 mA. Biases beam dump        relative to cathode.    -   M1: measures electron current leaving electron gun.    -   M2: measures cathode emission current.    -   M3: measures electron current arriving at beam dump.

Another embodiment of the present invention is suited particularly forion implantation systems which extract ions from a slot between one andthree inches long. The embodiment provides an efficient design for thegeneration of high currents of ions (e.g., 5 mA of each dopant beam isachievable). In this design, a filament approximately the same length asthe ion extraction slot provides a one-dimensional “sheet” of low-energyelectrons. The filament is oriented parallel to the ion extractionaperture slot, as also indicated in FIG. 6. An electron beam, ascontained within the first embodiment, is not required. This geometryhas two very significant advantages: 1) A high electron current isattainable, and 2) low electron injection energies can be achieved whilestill delivering high electron currents. A corollary to 2) is that,since ionization cross sections at approximately 100 eV are a factor of5 greater than, e.g., 2 keV, very significant ion currents can beachieved. 3) Finally, no magnetic field is used, and magneticconfinement is not necessary to keep the electron beam from diverging,since the electrons are extracted as a one-dimensional beam,significantly reducing space-charge effects.

Hence, FIG. 6 a shows a simple design for an ion source in which theelectrons are injected into an ionization chamber along the samedirection as the extracted ion beam. A long filament 170 is heatedthrough filament leads 171 and DC power supply 172 to emit electrons 173along the length of the filament. The filament may be a ribbon, or athick tungsten wire, for example. The filament 170 is biased below thepotential of the ionization chamber 175 by power supply a 172 such thatthe electrons are accelerated through a rectangular entrance slot 74centered in the rear of the ionization chamber 175. This constitutes adiode arrangement. A top view of this geometry is shown in projectionD-D, FIG. 6 b. The extended electron beam will ionize the gas within theionization chamber 175; the ions are extracted through ion extractionaperture 176 within the ion extraction aperture plate 177. Apart fromits simplicity, the advantage of the design of FIG. 6 a is that highelectron currents can be generated by the long filament 170 and focuseduniformly along the ion extraction aperture 176. The ion beam thusproduced should be uniform, since the electron path length through thegas within the ionization chamber 175 is the same along the length ofthe ion extraction aperture 176. Also, since the electron beam iselongated in the vertical dimension, it is less susceptible to spacecharge blow-up, and thus higher total electron currents of a givenenergy can be delivered into the ionization chamber 175 than with asmall, round electron beam.

To improve performance, a grid electrode 179 with a long rectangularslot can also be inserted between filament 170 and chamber entranceaperture 174 to improve focusing of the electron beam. This constitutesa triode configuration. To prevent the possibility of any transitionmetal contamination of the ionization chamber due to evaporation of thefilament onto the entrance aperture 174 and eventual migration oftungsten or rhenium into the chamber 175, the filament can be furtherremoted, removing evaporated material from line-of-sight with theionization chamber.

This embodiment is illustrated in schematic FIG. 7 a. The filament 170is remoted away from the ionization chamber 175, and the electron beamis propagated through a lens comprised of a series of long, rectangularapertures. The schematic representation of FIG. 7 a shows a triodearrangement in which filament 170, beam shaping electrode 178, gridelectrode 179, and ionization chamber entrance aperture 174 are all heldat different potentials, but this arrangement is not limited to atriode; more lenses can be added as needed. A top view, showing ingreater detail the electron optics of FIG. 7 a and the propagation ofthe electron beam 173, is shown in FIG. 7 b. Typical electrode voltagesare: cathode potential V_(C)=−100V, beam shaping electrode V₁=−102V,grid potential V₂=+100V, source potential V_(S)=0 (all voltages areshown relative to the ionization chamber, or source, potential), and ionextraction electrode potential V_(E)=−5V, for example, although the lenssystem V_(C) through V_(S) can be operated over a broad range ofvoltages and electron energies to optimize performance in producing theions of interest to a particular implantation process. V_(E) is a biasvoltage on the ion extraction aperture 177, which establishes a constantdrift field E as also indicated in FIG. 7 b. E imparts a drift velocityto the ions created within the ionization chamber, attracting positivelycharged ions toward the ion extraction aperture plate 177, where theycan be efficiently extracted by an external extraction field to form theion beam. Since the ions are created along the electron beam 173, Etends to provide forward momentum to the ions, such that their lateralcomponent of kinetic energy is essentially thermal, i.e., of magnitude<<1 eV. FIG. 8 shows the triode filament injection embodiment of FIGS. 7a and 7 b incorporated into an ion source similar to that depicted inFIG. 3. FIG. 8 shows a detail of the ionization chamber 175 and triode200, and shows the triode contained within the source block 35. Theother elements of FIG. 3 such as low-temperature vaporizer 28, gatevalves 100 and 110, mounting flange 36 and gas feed 33, for example, arepart of this embodiment as well, although not shown. However, theelectron gun 42, magnet coils 90, and beam dump 70 are not part of theembodiment of FIG. 8, since these electron gun-related components havebeen replaced by the triode 200. Some further advantages of the approachof FIG. 8 are: 1) the filament is remoted to a lower pressure location(by directly exposing it to ambient vacuum within the source vacuumhousing through vacuum conductances 190, for example), enhancingfilament life; 2) remoting of the filament prevents contamination of theionization chamber by the filament material; 3) the lens systemfacilitates accel-decel transport of the electron beam, enabling higherelectron currents to be achieved within the ionization chamber. 4) Theradiative heat load produced by filament 170 is conveniently conductedaway in large part by the water-cooled source aluminum block 35. 5) Theelectron beam can be made to focus on the ion extraction aperture,resulting in a high ion extraction efficiency, and small lateralmomentum. This last is due to the fact that the nominal electrontrajectories are along the drift field E in FIG. 7 b, such that the ionswhich are created by electron impact reach the ion extraction aperturehaving essentially a thermal lateral component of kinetic energy, i.e.,<<1 eV. 6) The embodiment of FIG. 7 b and FIG. 8 results in alow-emittance, high-brightness source of ions, enabling improved controlof ion beam propagation through the implanter, and much improved spatialand angular uniformity of the ion beam on the wafer substrate, relativeto the prior art.

FIG. 7 c shows a tetrode geometry similar to the triode of FIG. 7 b, butalso having dimensional information. Dimensions are given in mm. Thetetrode enables true zoom capability, so that the focusing properties ofthe lens system are somewhat independent of the final electron energy.This allows for extraction of the electrons emitted from the refractoryfilament 170′ at a higher energy than the electrons entering theionization chamber 175′, i.e., using deceleration to deliver higher,space charge-limited electron currents into the ionization region.Representative lens voltages are shown in Table A, given for an objectdistance of 4 lens aperture diameters (“D”) and an image distance of 6Dfrom the reference plane of the tetrode. These lens tunings inject 100eV electrons into the ionization region by extracting the electrons atenergies ranging from 300 eV to 100 eV. These tunings assume that theelectrons have 100 eV upon entering the ionization chamber. Since theselenses are one-dimensional, they do not focus or confine the beam in thelong (y-direction) dimension of the slot. At high current density, theelectron beam expands along y due to space-charge repulsion, resultingin beam loss through vignetting. Space charge repulsion and beam losscan be much reduced by allowing positive ions to be produced along thebeam path by electron-impact ionization. The positive space charge ofthe ions compensates for the negative space charge of the electrons,reducing the net coulomb repulsion between electrons within the beam,thus reducing beam blow-up and enabling higher electron currents thanotherwise possible. In order to prevent ions created in the ionizationregion 181′ from being extracted an lost into the tetrode, lens element180′ is always maintained at a substantially positive potential relativeto V₀, the ionization chamber potential.

The creation of positive ions in the beam path is an effective means forreducing space charge forces: since the ions are heavy and slow-moving,their depletion rate is low, and a reasonable charge balance can bemaintained if the rate of ion production is similar to the ion lossrate. The ion production rate at any point in the electron team path isproportional to the local pressure of ionizable gas at that point. Forexample, consider local gas pressures P1>P2>P3>P4 shown in FIG. 7 c (Inote that it is possible to control the local pressures Pi by tailoringthe conductance between individual lens elements and the ambientvacuum). Further consider the predominant gas species being decaborane(B₁₀H₁₄), a large, heavy molecule with a large electron-impactionization cross section for the production of positive ions. Within theionization chamber 175′, P1 is highest, approximately 10⁻³ Torr, so thatspace charge compensation is very effective; ideally, the charge densityof ions is the same as the electron density. This ensures uniformpropagation of the electron beam within the primary ionization region,ensuring good uniformity of the ion density adjacent to the ionextraction aperture 176′, and hence a uniform ion density in theextracted ion beam. P4 is the ambient pressure of the source vacuumhousing (nominally 10⁻⁵ Torr or less), and because the gas within theionization chamber 175′ propagates through electron beam entranceaperture 174″, a large pressure gradient is established between theseextrema. Since the deceleration lens 153 is close to 45′, P2 isrelatively high, and space charge repulsion is reduced substantially.The region P3 is adjacent to the cathode 143′, hence it is desired tomaintain P3 close to P4, i.e., at a sufficiently low pressure that thearrival rate of decaborane molecules to the cathode surface is less thanthe desorption rate of decaborane byproducts which can deposit on thecathode surface. This is particularly important for low-temperaturecathodes such as LaB₆, and for field emitter cathodes. In general,refractory metal cathodes operate at sufficiently high temperatures thatdeposition of cracked process gases is not a problem.

In another embodiment, FIG. 9 shows the top view of a hybrid sourcewhich incorporates the design and operating features of the first twoembodiments. It is analogous to FIG. 7 b, but shows to intersection ofthe round electron beam 210 (shown going into the plane of the drawing)with the ribbon electron beam 173 from the long filament emitter-basedtriode 200. Thus, this third embodiment has both an axially-positioned,long filament emitter and a longitudinal electron beam. FIG. 10 shows adetail of the present embodiment incorporated into the ion source, whichis a modification of FIG. 3. In addition to the low-temperaturevaporizer 28 shown in FIG. 3, FIG. 10 further incorporates ahigh-temperature vaporizer 220 enclosed by the source block 35.Vaporizer 210 is positioned within source block 35 such that it does notinterfere with the vapor conduit 32 or the gas feed 33 of FIG. 3. Vaporis conducted from vaporizer 220 to the volume within ionization chamber175 by vapor conduit 225, shown in FIG. 10. The purpose of this secondvaporizer 220 is to introduce vapors from solid dopant compounds such aselemental P, As and Sb, and also Sb₂O₃ and InCl₃, for example. Thus,vapors of these and other commonly-used solid dopants, as well as vaporsof special low-temperature materials such as B₁₀H₁₄ and trimethyl indium(TMI) can be introduced into the ionization chamber 44 by the embodimentof FIG. 10.

The features and advantages of the embodiment of FIGS. 9 and 10 are: 1)high ion beam currents can be obtained; 2) lack of a magnetic field,combined with a high electron density near the extraction aperture, andthe creation of ions in a path along the drift field direction, resultin a very low-emittance source of ions. 3) A full complement of solidand gaseous feed materials can be used, and the incorporation of twovaporizers can accommodate both n- and p-type dopants without requiringservicing of the ion source between the different implants; 4) thelow-energy electron beam produced by the filament, in conjunction with ahigher-energy electron beam provided by the e-gun, enables theproduction of multiply-charged ion species through a stepwise ionizationprocess in which copious amounts of singly-charged species are producedby the low-energy beam, which are again ionized by the higher-energybeam, resulting in multiply-ionized species. 5) The ability to “tune”the electron beam energy of the several electron sources gives the ionsource of FIG. 9 great flexibility in producing different crackingpatterns of molecular feed species, allowing the tailoring of the ionbeam contents to specific implant requirements.

Drain Extensions

It is proposed that the technology disclosed herein has significantadvantages when applied to the process of implanting the drain extensionof a transistor that will result in a higher performance device. Thebasic concept is that the disclosed sources will provide a beam with lowemittance, which in turn will create a junction with reduced lateralabruptness, which gives the transistor higher performance in exactly theways needed for scaled technologies.

The transistor with a more abrupt lateral junction will have improvedperformance in several ways. First, the junction region contributes acomponent to the series resistance, and the more abrupt junction willcontribute less series resistance, which in turn increases the drivecurrent and transconductance of the transistor. The abrupt junction alsoreduces subthreshold conduction, which is very important as the supplyvoltage is reduced. One benefit of this feature is the reduction ofoff-state current, which would reduce the static current of the entirecircuit, extending battery life, for instance. Improved subthresholdcharacteristics also allow for more freedom in the overall design of thetechnology, allowing the reduction of the threshold voltage withoutincreasing static current. This allows a direct tradeoff between circuitperformance (improved by lower threshold voltages) and the standby power(which is improved by reducing static current). These features areincreasingly important as the supply voltage is reduced, as it is witheach generation of technology beyond 0.25 um.

Within the field of ion implantation, a beam of ions is produced andtransported to an impact target, with some degree of energy which causesthe ions to enter the target material and penetrate to some depth. Thisis shown diagrammatically in FIG. 11. The target is typically a siliconwafer 1 where a transistor or other structure is being fabricated. Theion beam 2 is directed at the silicon wafer 1 to intentionally place theions into the silicon into some feature that contributes to the creationof a functional device. The ions do not all travel along the same path,but rather create a distribution of atoms within the silicon, as shownby the depth profile 5 in FIG. 1. The depth profile is generallycharacterized by two parameters: the projected range 6 which is theaverage depth of penetration, and the straggle 7 which is a measure ofthe variation in depth of the atom distribution. These parameters dependstrongly on the conditions of the ion beam being used for theimplantation process, with heavier ions or those with lower energyproducing shallower profiles. Generally analysis of ion implantationprocesses only consider an ion beam 2 which is entirely parallel, whichallows direct computation of the expected profile, projected range andstraggle. However, ion beams are not entirely parallel. There alwaysexists some fraction of the beam that is not parallel, such as theillustrated nonparallel ion 4. Nonparallel ion 4 will impact the siliconsubstrate at a nonzero angle of incidence 3. In general, all ion beamscontain non parallel components, the magnitude of which depend stronglyon the beam conditions, the implantation equipment details, and thetuning of the implanter. Some of the implications of the various ionangles contained in the ion beam are discussed below.

There are several ways to characterize the non parallel components ofthe ion beam. First, as the beam 10 is created and leaves the ion source9, the emittance 8 can be measured to characterize the angulardistribution of beam 10, as illustrated in FIG. 12. This parameter is ameasure of the total angular distribution of the beam as it is extractedfrom the source, and is usually expressed in terms of the solid angle.Once the ion beam is being transported down the beamline, theterminology generally used to discuss the angular distribution is thedivergence 11. Divergence 11 refers to the maximum angle of the beamrelative to the beam axis. When the beam reaches the target, each ionhas an angle of incidence as described above, because the target mightbe tilted relative to the beam axis for specific process effects. Thus,beam divergence 11 generates a range of incident angles when the beamreaches the target. It is important to note that these terms andparameters are limiting: the beam actually contains ions traveling atmany angles, and there is a distribution function which would describethe beam density as a function of divergence angle. Still, these termsare useful because much of the following discussion relates to the ionswith the largest angles.

One exemplary ion implantation process discussed herein is the drainextension implant. This is one step in the formation of a transistor:see FIG. 13. The drain extension implant is very important because itforms the structures that define the most important aspects oftransistor functionality. First, it is the means by which self-alignmentis accomplished: the gate electrode 13/gate oxide 12 stack is patternedto create a well defined gate edge 14. The feature masks the ion beam 2which is going to form the drain extension. Where the gate stack hasbeen removed, the implant penetrates into the silicon and forms animplanted layer which is the drain extension 15, but where the gatestack has not been removed the implant penetrates into the gateelectrode 16. This process results in the drain extension junction beingaligned with the gate electrode, which makes a good transistor structureand is known as a self-aligned gate. As further discussed below,nonparallel ions 4 and especially those that are directed under the gateand impact the silicon at the base of the gate edge, like the ion shownin FIG. 3 are of particular interest.

An important aspect of semiconductor technology is the requirement forconstant scaling. Scaling is the process by which all dimensions arereduced so that more transistors can be placed in a given silicon area,reducing the cost per function. For ion implantation, the result is aconstant need to reduce the ion energy, because reducing dimensions alsoincludes reducing the ion depth dimension, and this is accomplished byreducing the implant energy. Notably, many aspects of semiconductortechnology must continuously develop new methods, equipment andmaterials to keep up with the industry demands for scaling, and ionimplantation is included. This scaling has progressed sufficiently tocreate a major issue for ion implantation: challenge to make thejunctions shallow enough to meet scaling requirements. The most severeissue relates to the formation of the p-type junctions since the boronatoms used are light and tend to penetrate deep into the silicon. Inparticular, the p-type drain extension is the most challenging implantbecause it uses the lowest energy boron beam. It is noted that there arefundamental problems with ion implanters delivering high current beamsat low energy, and the methods being used to improve the low energyboron beam currents have adverse effects on the quality of the beamdelivered, especially its divergence.

The ion implantation is not the only step required to form the drainextension. In addition, a heat treatment or annealing step must beperformed to make the implanted atoms electrically active. Anotherproblem is that this heat treatment must be performed at a hightemperature (i.e. >900 C) such that there are also diffusion effects toconsider. Diffusion is the movement of implanted ions out of theiras-implanted depth profile, and generally deeper and laterally withinthe substrate. Since the challenge is to make a shallow junction(actually Ultra-Shallow Junction, or USJ), diffusion effects must beminimized. There is also an additional issue as the diffusion time isminimized: an effect known as transient-enhanced diffusion (TED). Thiseffect is a result of silicon interstitials present in the implantedlayer due to the implantation damage to the silicon crystal caused bythe ion implantation. The result is a strong enhancement of the borondiffusion, which only lasts a short time. However, a short anneal cycleis desirable to minimize diffusion, and so the TED effect increases theneed to shorten annealing time. It is noted that diffusion, and TED,move the implanted layer both in depth and laterally. There is muchdevelopment of advanced annealing equipment and processes to performthis critical process, and whatever is developed to keep the junctionsshallow will also provide a benefit in improving the lateral abruptness.

The scaling of the boron implant energy has produced a crisis for boronimplantation: low productivity due to low current delivery capability.Since the need is for high ion currents at low ion energy, the result isa situation where the beam has high space charge density, which createsproblems. At beam extraction from the source, the space charge densitytends to compensate the extraction field and results in the well knownE^(3/2) relationship between beam current and energy. This effectdramatically reduces the available beam current as the energy isreduced. In addition, the space charge of the beam during transporttends to create a Coulomb force which pushes the ions laterally out ofthe beam, resulting in “beam blowup” and a loss of beam current as thebeam is transported down the beamline. This effect is also stronglyenergy dependent, with the result that it is very difficult to deliverLow Energy Boron (LEB) ion beams to the silicon wafer target.

There have been two approaches to addressing the issue of LEB beamcurrent by the implant industry, both of which result in a moredivergent beam at the wafer. The first approach is to design a beamlinewith the shortest possible distance between the source and the siliconwafer, which allows more of the beam to reach the wafer. These shortbeamline are also made with a more open transport bath, so that a largerbeam is able to transit the length. In terms of the beam divergence,this approach results in higher divergence of the beam at the wafer. Thesecond approach to increasing LEB ion current is the use ofdeceleration. In this approach, the beam is extracted and transportedmost of the way to the wafer at higher energy, and then the beam isdecelerated right before the wafer to implant at the correct energy.This approach also results in higher divergence at the wafer, and alsointroduces energy contamination to the beam on wafer.

The most important part of the depth profile is at the junction edge, asshown in FIGS. 14 a and 14 b. As illustrated, the implanted boron layerinteracts with the existing doping concentration to form a P/N junction.The gate edge and drain extension layer is shown in FIG. 14 a, where alateral cut is shown (A-A′ 17) which is below and parallel to thesilicon surface. In FIG. 14 b, the doping concentrations are shown alongthe A-A cut. In the region of the drain extension, the boronconcentration 18 is high, of order 1E20 cm-3, which is shown as a highvalue on the log scale 19. Prior to the LEB implant for the drainextension, there was an N-type doping concentration 20 alreadyestablished, which is shown as Phosphorus, but could be any kind ofN-type dopant. The critical feature is the spot where the two dopingconcentrations are equal, which is the junction edge, 21. The value ofthe concentrations is much lower here, since the N-type concentration ismore likely to be of the order E17-E18 cm-3. Thus, the boronconcentration 18 is also of this magnitude at this spot. It is now notedthat the boron concentration at the junction edge is much less than theconcentration within the drain extension, by at least 100X. This isimportant because is shows that a small component of the beam, of order1%, can be very influential in determination of the junction edge, aswill be further described below. In addition, the slope of the boronprofile in the lateral direction at the junction edge is defined as thelateral abruptness of the drain junction. This parameter is usuallyexpressed in units of nm/decade, or the lateral extent of the profile,in nm, required to move up one order of magnitude in boronconcentration. A typical value of the drain lateral abruptness, for 0.18um technology, would be 10 nm/dec, while requirements for future scalingreduce with each generation with a goal of <5 nm/decade.

A low divergence beam, such as that of the present invention, when usedto form the drain extension, will improve the lateral abruptness of thedrain extension and result in a higher performance transistor. Twomechanisms contribute to this advantage: reduced lateral penetration andreduced lateral straggle.

The first component, lateral penetration, is strictly geometric.Referring to FIG. 13, a beam with low divergence essentially places allof the incoming ions within the drain extension region, out from underthe gate edge, which is desired. A divergent beam will have nonparallelions incoming, and we are particularly interested in those nonparallelions positioned exactly as depicted in FIG. 13, where the spot where theion penetrates the silicon substrate is right at the base of the gateedge 14. This ion, on average, will come to rest at the position markedwith the X, which is laterally displaced from the primary distributionof drain extension boron atoms. Since there is a distribution ofincident angles within a divergent beam, the result is a lateralextension of the profile under the gate edge. This produces a gradedjunction in the lateral direction which is not as abrupt as that whichwould be produced by a low divergence beam. As discussed above, theconcentration at the junction edge is much lower than the peakconcentration in the drain extension, so it only takes a small fraction(1%) of the beam to have high divergence to significantly extend thejunction laterally. In addition, there generally is a distribution ofangles within the beam where the higher angles have less intensity, alsocontributing to a laterally graded junction. For an estimate of thedegree of improvement with a low divergence beam, we can estimate thelateral displacement by the sine of the incident angle times the depthto the vertical junction edge. If we use 7 degrees for our divergentbeam, the lateral displacement is 12% of the junction depth, while thenormal estimate is the lateral junction edge is at 70% of the junctiondepth. Since this would now place the lateral junction at 82% of thevertical junction depth, the divergent beam produced a junction 17% moreextended than a love divergence beam would (for this effect).

The second mechanism involved with beam divergence and lateral junctionformation is the lateral straggle. Straggle occurs because the silicontarget is not a uniform homogenous medium and is rather a crystallinelattice with individual atoms arranged in a regular pattern with spacein between. The incoming ions may either hit a silicon atom directly,hit a silicon atom with a glancing incidence, or miss the silicon atomscompletely. This statistical process results in a distribution ofconditions for the various incoming boron ions. The straggle is normallyconsidered as a vertical variation in the depth profile, but a maskededge like our present case also involves lateral straggle. The importantfactor is that the lateral straggle is dependent on the incident angle,with divergent ions producing more lateral straggle. For a more detaileddiscussion of this phenomenon, the reader is referred to reference:Nakagawa, Hada and Thome, IIT '98, p 767. One of the figures from thisreference is reproduced as FIG. 15 illustrating the results ofcalculations of the lateral straggle as a function of the incidentangle, θ, and the ion energy. The normal expectation of 70% lateral tovertical straggle is shown as the dashed, relatively horizontal line,which is explicitly noted as assuming a zero angle of incidence. Thedata points are for various incident angles and energies, but it is seenthat it is quite likely that ions incident with an angle of 7 degreeshas at least double the lateral straggle of the conventional model. Itis also noted that the effect is larger for higher angles, so thedistribution spreads out laterally, which is exactly the opposite of thedesire to keep the junction abrupt.

Referring now to FIGS. 16 a and 16 b, in order to make numericalestimates of the relative magnitudes of these effects, a model isconstructed which resolves these effects into the same terms. The firststep in the model is to reduce the variables by making an approximation:the vertical junction depth is equal to the projected range (Rp) plustwo times the vertical straggle (ΔRp) orXj=Rp+2ΔRp

This allows the expression of the lateral junction position in terms ofthe vertical straggle, since the relationships have already beenexpressed.

The low divergence lateral junction edge occurs at a position which istwo times the lateral straggle by this model. Since we already know thatthe lateral straggle is 0.7 times the vertical straggle, the lateraljunction edge occurs at 1.4ΔRp inside the gate edge. Now, the divergentbeam case includes two terms, which add to produce the lateral junctionedge. The first term is that the lateral straggle is twice the normalincidence case, so the this contribution is 2.8ΔRp. The second term isthe geometric effect which was 12% of the junction depth, which is now0.48 ΔRp. Adding this to the other term produces a lateral junction edgewhich occurs at a position 3.28ΔRp inside the gate edge, or 2.3 timesmore extended in the lateral direction. The lateral abruptness will alsobe improved by a similar ratio. This is a dramatic advantage whichdirectly results from having a source with low emittance.

Another critical parameter associated with an MOS transistor is thechannel length. The channel length is the distance between the sourceand drain, that is, between the lateral junction edge of the source andthe drain. While the discussion so far has centered on the drain side ofthe transistor, there is another region on the other side of the gatewhich forms the source at the same time that the drain is formed. It isimportant to note that it is the lateral extension of the source anddrain that determine the channel length. The implantation profile isdetermined by the implantation parameters, most importantly species andenergy, but also tilt, twist, mask edge, and beam divergence. Onceagain, the beam emittance determines the beam divergence, and thus hasan impact on the transistor formation. It is clear that the channellength will be longer for the low divergence beam, since there is lession beam penetrating under the gate stack. It is not a benefit to havelonger channel lengths in general, but the low divergence case producesa transistor closer to the ideal situation where the channel length isthe same as the gate length.

For this case, the problem with the high divergence beam is that thedivergence is not always the same. This is a natural result for beamconditions with significant divergence, because of the natural variationwith beam tuning and setup. The variation comes because any time thatthe beam intersects an aperture, some of the beam is lost, and part ofthe divergence envelope is also lost, because there is a directrelationship between the position in space and the angle that that partof the beam will have with the substrate when it gets there. Forexample, take two conditions: one with the beam setup centered on anaperture and one where the aperture clips part of the beam. For thecentered case, the divergence will also be centered, and so the beamintersects the wafer with the nominal angle plus or minus the sameamount, say +/−5 degrees. For the second case, the edge of the beam,which is clipped by the aperture, is also the extreme of the angularspread, which is only taken off of one side. So, in this case, the angleat the substrate might be the nominal angle with asymmetric divergence,say +2/−5 degrees. Now, if the side where the divergence has beenclipped is the side which is defining the edges of the transistorchannel, we have changed the junction profile by eliminating the highangle ions, such that the channel would now likely be longer because theions did not penetrate as far under the gate edge. This variation inchannel length is highly undesirable, and a beam with low emittancewould not be subject to this variability. It is noted that thevariations discussed are normal, and are a result of the automation ofmodern equipment. In today's production tools, an automatic routineestablishes the beam conditions, running a sequence of prescribed stepsto establish and optimize the beam conditions. It is normal that thissystem does not reach the same beam conditions each time; its task is tocreate a beam consistent with the requirements, and there are alwaysmany solutions to the tuning problem. These tuning solutions producedifferent divergences in the beam itself, it is not necessary to have anoff center beam, or any abnormal condition, to create beams withvariable divergence.

Another benefit of the low emittance ion beam would be the ability todesign the process so that the channel lengths are uniformly small,rather than having to design so that the shortest channel only occursfor worst case conditions. Since the performance of the circuit isdirectly connected to the channel length, the ability to produceuniformly short channels directly allows higher performance to berealized, for all other process steps being unchanged.

Note that channels too short are likely to fail, either because thedrain to source voltage cannot be sustained because the channel is tooshort, or that the threshold voltage falls out of the operational rangedue to short channel effects. So, in the process of designing theprocess and the circuit, the distribution of channel lengths produced bythe normal range of process variation must be conservatively considered,so that none of the transistors fail (failure rates of even one ppm aretoo large). Stated another way, the distribution of channel lengthsshows less variation, and this allows the average channel length to bedesigned to be smaller, resulting in higher performance at no additionalcost.

Extended Ribbon-Beams

There is currently great interest in extending the design ofconventional ion implanters to produce ribbon beams of larger extentthan heretofore. This interest in extended ribbon beam implantation isgenerated by several factors: 1) the recent industry-wide move to largersubstrates, i.e., 300 mm-diameter silicon wafers; 2) the expectationthat even larger substrates, i.e., 450 mm-diameter silicon wafers, willbe put into production for conventional CMOS and other devicemanufacturing, and 3) the recent industry-wide move towards serialimplanter designs which can benefit greatly from the incorporation ofelectromagnetically-scanned extended ribbon beams in order to increasewafer throughput, and to improve dose uniformity across the substrate.Since in conventional ion implant the wafer throughput tends to decreasewith increasing substrate size and is inversely proportional tosubstrate area, and further, since the expected economic benefitsinherent in the use of large-area substrates cannot be realized unlesswafer throughput is kept roughly constant, the ability to deliversignificantly higher ion beam currents is critical to the continuedsuccess of ion implant in silicon device manufacturing. While the beamcurrent (hence dose rate) delivered to the wafer can scale with thelength of the ribbon beam, this requirement is hindered by prior art ionsources for the following reasons: 1) prior art ion implantation sourcescan only produce ribbons of a limited extent (up to between two andthree inches long), and 2) if the extended ribbon beam is produced bybeam expanding optics, the current density in the beam drops inproportion to the magnification, such that the total current deliveredto the large substrate is unchanged.

By utilizing the technology I described previously in embodiment 1, 2,and 3, I can produce ribbon beams of almost arbitrary extent asextracted directly from the ion source. This is accomplished by simplyscaling the length of the ion source as indicated in FIG. 17. FIG. 17shows an embodiment similar to that of FIG. 4, namely an electron gun230 delivering a variable-energy electron beam 235 into a ionizationchamber 240 filled with dopant-containing gas, and intercepted by awater-cooled beam dump 250. The electron beam propagates parallel to,and adjacent to, ion extraction aperture 260 from which an ion beam isextracted by extraction optics. An optional external magnetic field B isprovided by magnet coils (not shown). The use of a longitudinal magneticfield oriented parallel to the path of the electron beam will confinethe electron beam 235 over even a very long path length. The path lengthis given by (x+y) as indicated in FIG. 17, where x is the extent of theelectron gun, and y is the extent of the ionization chamber (y is alsoroughly the length of the ion extraction aperture, and the desiredlength of the extracted ribbon ion beam 270). I envision the profile ofthe ionization chamber 240 to be cylindrical, with the extractionaperture occupying a flat face of the cylinder.

The arrangement of FIG. 17 can be advantageously used as an ion sourcefor the ion doping of fiat panel displays. For example, the ionextraction aperture 260 can be 850 mm long for producing ribbon beams toimplant rectangular panels having a short dimension of 750 mm. In thiscase, the ionization chamber lengthy in FIG. 17 would be longer than 850mm, for example 900 mm. Electron gun 230 is designed to deliver a highcurrent, low energy electron beam 235 into ionization chamber 240.Typical specifications are: cylindrical lens diameter=1 inch, electronbeam energy=100 eV (adjustable between 20 eV and 250 eV), maximumelectron current=200 mA, electron current dynamic range=400 (i.e.,electron current is adjustable between 500 μA and 200 mA). The electronbeam is confined both in the electron gun and in the ionization chamberby an external magnetic field B produced by a pair of magnet coils. Itis important that the electron beam be well-confined and collimated by Bbecause of the long electron beam path length through the ion source. Amagnetic flux density of between 50 G and 200 G is applied in order tomaintain good uniformity of ion production (ion density) across thelength of the ion extraction aperture 260 by limiting spreading of theelectron beam diameter as it propagates through the ionization chamber240 due to space-charge forces within the low-energy electron beam. Theuniformity of ion generation along the aperture is further improved byreducing the feed gas pressure (relative to typical prior art ion sourcepressure which ranges between about 4×10⁻⁴ Torr and 4×10⁻³ Torr) withinthe ionization chamber 240 so that a smaller fraction of electrons arescattered out of the beam, for example an ionization chamber pressure of1×10⁻⁴ Torr or less. We note that prior art, plasma-based ion sourcescannot operate at significantly reduced pressure since the plasma cannotbe sustained at low pressure. This low-pressure operating mode canreduce process gas consumption in ion doping systems by more than anorder of magnitude, significantly reducing tool cost of ownership (COO).

A further, dramatic reduction of COO is illustrated in FIG. 18 b, whichshows a dual ion source system 600 with dual-slit extraction optics 610.A single pair of large-diameter magnet coils 620 provides a uniformmagnetic field which encompasses both ion sources. The embodiment ofFIG. 18 b enables both n- and p-type dopants to be implanted in a singleion doping tool by dedicating n-type (e.g., phosphorus) materials insource 1, and p-type materials (e.g., boron) in source 2. Sources 1 and2 are not typically run at the same time. If desired, both ion sourcescan be run simultaneously with the same dopant, producing two ribbonbeams, doubling the implanted dose rate. FIG. 18 b illustrate two ribbonion beams being generated, beam 630 (e.g., boron-containing), and beam640 (e.g., phosphorus-containing).

FIG. 18 b shows the dual ion source of FIG. 18 a doping a rectangularpanel 650 with ribbon beam 660. In this illustrative example, panel 650is mounted on scan stage 670 and mechanically scanned along direction680, along the long dimension of the panel 650. Note that in this case,ion beam 660 is indicated as longer than the short dimension of thepanel.

FIG. 19 shows a generic ion doping system for the doping of flat panels.Panel 690 is loaded from vacuum cassette 700 into process chamber 710,rotated through 90 degrees, and scanned vertically in front of ion beam660′. The dual ion source 600′ of FIG. 18 a, 18 b is shown integratedonto the system in schematic. Magnet coils and details of mechanicalfeedthroughs are not included, for clarity.

The p-type feed gases of choice in ion doping are diborane (B2H6) andboron triflouride (BF3). Since there is no mass analysis between the ionsource and the substrate, all ions produced in the ion source areimplanted into the substrate. This makes the use of BF3 problematic,since fluorine is detrimental to oxides, for example, and hasundesirable process effects. Also, there is three times as much fluorineas boron in the source plasma, so much F can be implanted. In the caseof B2H6, which most manufacturers prefer to BF3 since there is not muchprocess effect from H implantation (H implantation causes excess heatingof the substrate, for example), there are two major complaints: 1)extensive cracking pattern (many different ions produced, for example,significant fractions of H⁺ and BH_(x) ⁺, as well as B₂H_(x) ⁺. Thisresults in a broad end-of range implantation due to the variety ofeffective boron energies implanted into the sample. 2) Insufficient beamcurrent resulting in low throughput, due to the fact that, in aconventional bucket-type source, most of the boron-containing ionsproduced are deposited on the walls of the ion source chamber.

The use of the ion source of FIG. 17 and the substitution of decaboraneas a feed gas material solves problems 1) and 2), since high currents ofa 70% pure B10Hx beam is produced (as shown in FIG. 19 a, aNIST-traceable spectrum of decaborane), the surface area of my ionsource is orders of magnitude smaller than in a bucket source, and theion source of FIG. 17 demonstrates a high ion extraction efficiency. Thenet result is that the use of the ion source of FIG. 17 runningdecaborane enables much higher throughput, much lower COO, and much lessparticulate formation (since less material is accumulated in the ionsource) than the prior art.

Acceleration/Deceleration Techniques

The production of high-brightness ion beams is very important in ionimplanters which employ deceleration of the ion beam prior to its impactwith the substrate, since both the angular divergence of the deceleratedbeam and its spatial extent are increased after deceleration. To producea small angular divergence beam on the target substrate with goodspatial uniformity after deceleration, an initially low-emittance beamis required. Since the beam emittance (product of the beam diameters andangular divergences in two orthogonal directions) is inverselyproportional to energy, the emittance of the upstream beam must besmaller than that desired at the substrate by at least an amount equalto the deceleration ratio. While the beam emittance can always be keptbelow a given value by the addition of a series of apertures, theresulting beam flux is unacceptably low. Therefore, the use of ahigh-brightness ion source is desirable, where brightness is defined asbeam current divided by emittance (i.e., beam current per unit area perunit solid angle). The brightness is unchanged by such a series ofapertures, and hence is a useful figure of merit.

Certain production implants, such as creation of drain extensions,require both low angular divergence at the substrate and low energyions, which work against each other unless a high-brightness source isused. The net result is that a much higher implanted dose rate isachieved with the high brightness beam than with a beam of lesserbrightness. This directly leads to higher product throughput, and lowercost devices.

The use of ionized clusters, which contain a single charge but multipledopant atoms, enables higher brightness beams especially if we replacebeam current by dose rate, or “effective” beam current. Since asingly-charged cluster of n atoms must be accelerated to n times theenergy, the emittance of a cluster beam is n times smaller than that ofa process-equivalent monomer beam. Since the dose rate is also n timesthe electrical ion current, the total increase in brightness of a givencurrent of cluster beam is n², when brightness is defined as dose ratedivided by emittance. Thus, the use of a high-brightness ion sourcewhich can produce cluster ion beams is an enabling technology whichallows decelerated beams to perform well-controlled implants with smallangular divergence, good spatial uniformity, and high throughput.

In particular, FIG. 20, which is substantially the same as FIG. 1 ofabove-incorporated PCT Application Serial Number US00/33786 anddescribed more thoroughly therein, shows a general schematic of a decelimplanter such as is used with conventional boron implantation. FIG. 20describes a conventional, non-decel implanter. For example, the ionsource 548 produces ions which are extracted from a one-dimensionalaperture (i.e., an elongated slot) and accelerated to a transport energysignificantly greater than the desired final implantation energy by anelectrode 553, and are injected into analyzer magnet 543 which dispersesthe beam laterally according to the mass-to-charge-ratio of the ions. Amass-resolving aperture (slot) 544 allows only the ion of interest (theion having a preselected mass-to-charge ratio) to pass downstream to amoveable Faraday for measuring ion beam current, or (when the Faraday isretracted) to the deceleration electrode 557. The deceleration electrode557 decelerates the ion beam to the desired implantation energy, whichthen impacts the wafer substrate 555. The schematic of FIG. 1illustrates a batch-style implanter with a mechanically rotating andscanning disk 545, but the general approach of decel can also be adoptedin serial implanters.

Since there is a finite probability that some of the ions in the ionbeam will undergo charge-exchange interactions with the residual gasmolecules in the implanter beam line or with other ions in the beamprior to reaching the decel electrode, most acceleration/decelerationimplanters also incorporate a neutral beam filter (not shown in FIG. 20)or other type of energy filter (for example, E×B filter, electrostaticdeflector, dogleg, etc.) to make sure that only ions of a predeterminedenergy reach the wafer, as is known in the art.

Typically, a batch style decel implanter will utilize a stationary ionbeam, the scanning of the beam across the wafers being accomplished bythe rotating and mechanically scanning disk 545; however, otherembodiments are possible. For example, the present invention can, withadvantage, be incorporated in a serial-style implanter (one wafer at atime) which accomplishes fast scanning (by either electrostatic scanningplates, or by a directional magnetic field) in one direction, while thewafer holder accomplishes a slower mechanical scan in the orthogonaldirection. Alternatively, dual electromagnetic scanning of a stationarywafer is also possible. Serial-style decel implanters have never to myknowledge been commercialized, I realize such a design would havedistinct advantages by being able to accommodate single-waferprocessing, as well as enabling high tilt angles (up to 60 degrees) ofthe wafer holder (not currently possible with batch implanters). Hightilt implants can be important in many processes, being preferred, forexample, in “quad” implants for the fabrication of well structures, andfor profiling of the S/D extensions. In addition, newer waferfabrication facilities are expected to adopt single-wafer processing inthe future to reduce the risks to expensive 300 mm process wafersincurred by batch processing.

Serial implantation requires a much higher degree of uniformity of thebeam profile than in batch-style implantation to maintain gooduniformity of the implant across the wafer, and this requirement becomesmore difficult to achieve for 300 mm versus 200 mm diameter substrates.A feature of my invention is combining cluster beam implantation withthe acceleration/deceleration technique by a serial, high-current beamline implantation system, the improved profile of the beam, asdescribed, meeting the beam profile uniformity demands needed for theserial implanter to be production worthy.

FIG. 21 shows one preferred embodiment of the ion source used in thepresent invention, mounted onto the vacuum housing of a conventional ionimplanter. This ion source is fully described in the above referencedPCT Application Serial Number US00/33786, in which it is included asFIG. 9B. The ion source operates on a different principal thanconventional ion sources presently in use in commercial ionimplantation, in that the ions are produced not by an arc discharge orby a plasma, but by direct electron-impact ionization by a broaddirected beam of energetic primary electrons that transit an ionizationvolume 516′. This ion source is an enabling technology for ionizingmolecular compounds, and provides high currents of decaborane ions, aswell as of the dimer-containing compounds of Table 1a. The sourceincorporates a low-temperature vaporizer 528 for producing vapors fromsolid materials having a low melting point such as indium hydroxide,trimethyl indium, and decaborane, for example, and also incorporates agas feed 526 which allows the ionization of gaseous compounds such asPH₃, AsH₃, GeH₄, B₂H₆, as well as more common implanter gases such asBF₃, SbF₅, and PF₃. The embodiment of FIG. 21 shows an extended electrongun which incorporates a 90 degree bending stage or mirror 587 to reducethe footprint of the source assembly for retrofit into existing ionimplanters, and to conserve space while enabling a full complement ofelectron optics to be incorporated into the electron gun to achievevariation of the electron energy to match the ionization needs of theselected molecular species.

FIG. 23 shows another different embodiment of an ion source and it'selectron gun, which is also fully disclosed in the above-referencedapplication. The electron gun operates, itself, on anacceleration/deceleration principle, and does not use a turning stage.FIG. 23 shows the ion source mounted in a volume of the vacuum housingsimilar to FIG. 21, but with the housing modified to accommodate thestraight-through electron gun design. The embodiment of FIG. 23 also hasa set of magnet coils to provide confinement for primary electronswithin the ionization volume of the ion source. High electron currentsat a controllably variable electron energy can be is injected into theionization chamber by the electron gun; the majority of electrons whichtransit the ionization volume are intercepted by a beam dump 536′ (FIG.22).

In the embodiment of FIG. 22, the length of the ionization chamber andthe corresponding elongation of the ion extraction aperture is greatlyextended, such extended length of the ionization chamber having littlenegative effect and a very positive advantage on the successfuloperation of the ion source. This great elongation is in sharp contrastto the conventional Bernas-style arc discharge ion sources commonly usedin ion implantation. In these the arc becomes unstable if the chamberlength (and hence the separation between the cathode and repeller) ismade significantly longer than the common length while the arc currentrequired to operate the Bernas source would increase dramatically (aconventional Bernas source has a cathode—repeller separation of about 2inches, and draws up to 5 A of arc current). By greatly extending thelength of the ionization volume and the ion extraction slot aperture,according to the present invention, more ion current can be extractedthan in the previous embodiment. Special optics are provided,constructed to reduce the length of the beam profile thus produced asthe beam progresses away from the aperture. This approach is illustratedby FIG. 24. Elongated ionization chamber 500 has an elongated ionextraction aperture 510 from which the ion beam is extracted andaccelerated by extraction lens 520. I presently contemplate theextraction aperture 510 to be about six inches in length, three timesthe length of the extraction slit of a conventional arc dischargechamber of a high-current ion implanter. Extraction lens 520 has thespecial property of being telescopic; it is a two-stage accelerationlens in which the second focal point of the first lens and the firstfocal point of the second lens roughly coincide, enabling telescopicfocusing. By constructing the lens 520 to achieve a three-to-onedemagnification, the height of the ion beam is substantially reducedwhile preserving a well-collimated beam trajectory 530 for injectioninto the implanter's analyzer magnet 540. Heretofore such an ion opticalapproach would be unsuccessful, since if the ion extraction aperture ofa high-current implanter were made much longer to increase extractedbeam current of a monomer beam, space-charge forces would not permitlens 520 to function properly, and the beam trajectory 530 would not bewell-collimated; that is, the beam would simply blow up. In fact, thenet result would be to extract and inject into the analyzer magnet lessuseful beam current than with a conventional, two-inch aperture! Thereason is that in a conventional Bernas arc discharge source used in aconventional high-current implanter, beam transport is fullyspace-charge limited at extraction, since up to 50 mA of ion current (ina BF₃ plasma, about 30% or up to 15 mA is boron, B⁺) can be readilyproduced from a two-inch aperture. The ion source of the presentinvention, in contrast, can produce 1 mA or more of ion current from atwo-inch aperture, of which about 70% is decaborane, so that even from asix-inch aperture, between about 3 mA and 5 mA of ion current should beobtainable. Since this low ion current can be extracted at a much higherenergy than possible in a decel implanter using boron (the decelerationratio is not arbitrary, but is selected to be within desired energycontamination, and transport limitations, and to achieve desired levelsof beam current), it follows that the transport depicted in FIG. 24 isnot space-charge limited. Hence, by elongating the extraction apertureof the ion source of the present invention as depicted by FIG. 24, it isanticipated to be possible to triple the beam current, and yet maintaingood beam emittance properties.

Electron Gun

As an introduction, to the electron gun construction now to bedescribed, it is important to observe that using hot cathode basedelectron guns of the type disclosed is problematic in a vacuumenvironment since radiative loads from the hot cathode and alsoconductive heat transfer between the cathode lead and the surroundings,and in particular electron bombardment of the individual lens elements,all conspire to make heat dissipation a significant problem in lensdesigns which isolate the lenses from objects which are at thermalequilibrium with a cold reservoir. Efforts to create electricallyisolating and thermally conductive mechanical paths to conduct heat awayfrom the individual lens elements, the most important of which would bethe cathode assembly since it dissipates much of the heat which isdissipated throughout the electron gun, are also highly problematicsince the mechanical construction of electrically insulating andthermally conductive paths to a cooled heat reservoir are difficult toachieve in practice and are prone to failure. One possible solution tothis problem is to allow the lens elements to float to whatevertemperature allows them to be in thermal equilibrium with theirsurroundings, however, this approach, too, is problematic. Even if onewere to construct the lens elements out of refractory materials thatcould easily maintain an operating temperature of 1000° C. or more,interactions with process gas make this an unsatisfactory solution. Inparticular the use of decaborane in such an environment would causecracking of the decaborane upon contact with lens elements anddeposition of boron onto the lens elements creating particulates whichare deleterious to the implant process in general, could createelectrical shorts by the coating of insulators and can also reduce thecathode and ion source lifetime significantly. According to theinvention, an elegant temperature control arrangement is achieved byradiative heat transfer and vacuum to a cool body which subtends nearly4π steradians surrounding the lens elements and more specifically heattransferable holders which also accomplish precise registration of thelens elements with each other and also with the ion source per se.

Referring to FIG. 25, a lens element 300 is held by clamping holder 310.Holder 310 is comprised of an elongated rectangular cross-sectionaluminum rods having large radiative surface which grasps lens 300 via aclamping arrangement. In the embodiment shown, the lens 300 is insertedinto a bore by spreading of the clamp through the insertion of aspreading tool, not shown. Alternatively, clamp 310 and lens 300 may bejoined through extreme cooling of lens 300 to shrink it to a smallersize than the bore, for example by immersion in a liquid nitrogen bath,and insertion of the cold, reduced diameter lens 300 into clamp 310,subsequently allowing lens 300 to expand to room temperature, thus coldwelding the assembly together. These means of mounting and holding lenselements such as lens element 300 produces effective heat transferacross the mated, contacting surfaces and enables radiative heatdissipation from the large surface area of clamp holder 310. To improvethe emittance properties of the aluminum holder 310, the services can beanodized or coated with a colloidal suspension of carbon such asAquadag®. An additional benefit of the assembly of lens holder 310 andlens element 300 is that the holder and the lens element may be ofdifferent material. For example, lens 300 may be constructed ofstainless steel which is chemically inert or molybdenum which has verygood structural characteristics at high temperature.

Referring to FIGS. 26 and 27, the electron gun consists of four discreteand separated lens elements: the cathode assembly 320 followed, insequence, by first anode 330, focus electrode 340, and exit lens 350,each lens in array to be at different potential held by a respectiveholder of the general construction just described for holder 310. A gapis provided between lens element 350 and the base of a housing 360 whichcompletely surrounds the assembly of four spaced-apart lens holders. Thehousing 360 of aluminum maintained at much colder temperature than lenselements 320-350 and allows radiative coupling between the several lensholders 370. This is accomplished by close coupling the base 360 a ofhousing 360 in good thermal contact with a water cooled or temperaturecontrolled source block which has been previously described.

Because of the large contact surface area and the good thermal coupling,which is accomplished by thermally conductive elastomeric seals betweenbase 360 a and source block which is temperature controlled, the housing360 can be maintained at a temperature not too different from thetemperature of the source block. Thus, a temperature difference ismaintained between the several lens holders 370 and the housing 360,enabling good radiative transfer from the broad radiative surface of thelens holder. Furthermore, the use of radiative cooling enables a stableoperating temperature for the electron tin, that is somewhat independentof power dissipated in the gun elements. This stability is due to thenonlinear effect of radiative cooling which is much less efficient atlow temperatures than at the very efficient high temperatures. Thus theassembly is, to a degree, self regulating and allows for consistentoperating temperature of the electron gun elements.

The several lens holders 370 are constructed in a mostly rectangularprofile so that significant surface area is exposed both to neighboringsurfaces of the other lens element holders as well as to the surface ofthe housing 360. This arrangement accomplishes two functions. The firstfunction is that the regions where the highest power dissipation isexpected (namely the uppermost lens element comprising the cathodeassembly 320 and the bottommost lens, exit lens 350) are directly cooledby close proximity to the heat reservoir represented by housing 360,whereas some of the power dissipated in the lens element 330 and 340shown in FIG. 26 is distributed among the lens elements, enabling a moreuniform operating temperature to occur. Nevertheless, there issignificant radiative loss area exposed to the cooled housing 360 forall the lens elements since four sides of the holders are radiativelyexposed to opposing surfaces of the housing. If two or fewer sides of aholder are exposed to neighboring elements and two or greater sides areexposed to the cooled housing 360.

Referring now to FIG. 25, typical dimensions of holder 310 are a lengthl of 170 millimeters, width w of 26 millimeters, and height h of 12millimeters. Lens element 300 typically has an inside diameter of 12.5millimeters and an outside diameter of 16 millimeters.

Turning now to FIG. 26, the overall dimensions of the gun assembly areshown with length L of 6 inches, height H of 3 inches, and width W of1.5 inches. Feed through 380 contacts the individual lens elementsthrough metal springs indicated by 390 which are electrically isolatedfrom each other by a holder plate 395 of ceramic, which allowsmechanical stability of the clips such that when the clips are engagedto the several lens holders 370 they accomplish both electrical contactto the individual elements and also provide indexing and registration tomaintain alignment of the lens elements. The lens elements are furtherconstrained and insured to be coaxial by an alumina rod 400 which passesthrough several alumnus spacers 410 that are individually counterboredinto each of the lens holders 370. This accomplishes both electricalisolation and also controls the spacing between the lens elements andinsures alignment in three dimensions.

The lens elements 320, 330, 340, 350 and any subsequent lens must bemaintained in a coaxial relationship to a high degree of tolerance toensure proper focusing of the electron beam and to limit aberrations.The fields that are set up between the lens elements are very sensitiveto this alignment, especially in the polar angular coordinates definedby the cylindrical symmetry of the lens system (the z-axis being alongthe mechanical axis, i.e., the direction being the direction ofpropagation of the electron beam), and also the two dimensional spatialcoordinates that are transverse to this axis. However, it must be saidthat the degree of alignment required for the correct direction andfocusing of the electron beam is within the normal limits of machiningpractices and most importantly, of the alignment techniques accomplishedby alumna rod 400, individual spacers 410, precision machining of bothlens elements 320 through 350 and the several holders 370. Theconstruction of the lens elements in correct alignment ensures that thebeam will propagate in the correct direction through the desired volumein the ionization chamber and also that the beam will be well defined,propagating parallel to the long dimension of the ion extractionaperture. Thus, a small volume ionization region is precisely locatedadjacent to the ion extraction aperture to achieve high brightness andlow emittance, as described above. In addition, proper temperaturecontrol of the lens elements is important for the operation of the ionsource as a whole to prevent either condensation or decomposition ofprocess gas or process vapor which normally occupies the ionizationchamber and therefore penetrates into the electron gun region. Shouldexcessive decomposition or condensation occur in the lens elements itwill degrade the overall life time of the ion source and have a negativeimpact on preventive maintenance intervals. In order to ensure properion beam characteristics of the ion extracted from the ion extractionaperture it is important that the ionization region be uniformly locatedwith respect to the ion extraction aperture. Misalignment or defocusingof the electron beam will cause variation in proximity and size of theelectron beam and hence the ionization region along the long axis of theion extraction aperture which is undesirable. However, proper alignmentof the electron beam and proper focusing of the electron beam, asachieved by the alignment and coupling features, and temperaturestability which have been described for the holders, ensure that ionbeam created in the ionization chamber is bright, the transverse energyof the ions is limited, and the ions are created mostly located in frontof the aperture, and in a uniform density across the long axis of theaperture. The brightness of the ion beam that is extracted from the ionsource which is proportional to the total current and inverselyproportional to the emittance of the ions as they exit the ionextraction aperture, becomes higher as the ionization volume isdecreased as long as the total current remains constant. Thus with thedesign described, by achieving a dense electron beam which is wellcontrolled within a small ionization volume, a much brighter source ofions is obtained than would be obtained with a diffuse ionization regioncreated by a defocused or misaligned electron beam.

The brightness properties enabled by this lens and lens holder design,where the brightness of the ion source is maximized is very importantwhen used in an accel-decel (acceleration-deceleration) type ionimplanter where the performance of the implant on wafer is directlyproportional to the brightness of the ion beam. That is, thecharacteristics of devices formed by the implanter and also theproductivity of the implanter, are directly related to the level ofbrightness of the ion beam. Therefore this design enables highbrightness beams for accel-decel type implanters to be achieved and thusimproves the performance of accel-decel designs.

In the case of using decaborane or similar clusters to further enhancethe brightness of beams delivered in any kind of implanter and inparticular with respect to accel-decel implant ions, excellenttemperature control of all parts of the ion source including theelectron gun elements which are in contact with the decaborane vapor, asachieved here, is critical to the success of the ion source. Inparticular, it will enable high ion currents to be produced. It willalso enable much longer life time and much higher production worthinessin the final ion source and in the implants. Decaborane in particularwill dissociate to boron components when it meets a wall temperature inexcess of 350 C. Decomposed boron particles can deposit on cathodes andlens elements. If significant boron becomes deposited on the cathode itcan degrade the performance of the cathode and significantly reduce ionproduction, thus limiting lifetime of the cathode. Also boron componentscan cause lens elements to charge and become less effective atcontrolling the electron beam and hence reduce the brightness of the ionbeam generated by the ion source. Therefore, good temperature control asachieved by the design just described is critical to successfulimplementation of decaborane, particularly for accel-decel applications.

1. A method of ion implantation by producing a high brightness ion beamthat extends along an axis by ionizing molecules of a gas or vapor, themolecules containing an implantable species, the method comprising:providing an ionization chamber having a restricted outlet aperture,providing in said ionization chamber said gas or vapor at a pressuresubstantially higher than the pressure within an extraction region intowhich the ions are to be extracted external to the ionization chamber,by direct electron impact ionization by primary electrons, ionizing thegas or vapor in a region adjacent the outlet aperture of the ionizationchamber in a manner to produce ions from the molecules of the gas orvapor to a density of at least about 10¹⁰ cm⁻³ at the aperture whilemaintaining conditions that limit the transverse kinetic energy of theions to less than about 0.7 eV, the width of the ionization volumeadjacent the aperture, in which said density of ions is formed, beinglimited to a width less than about three times the corresponding widthof the outlet aperture; and conditions within the ionization chamberbeing maintained to prevent formation of an arc discharge, by anextraction system, extracting ions formed within the ionization chambervia the outlet aperture into the extraction region downstream of theaperture, thereafter, with ion beam optics, transporting the beam to atarget surface, and implanting the ions of the transported ion beam intothe target.